7t52

IntroductionThis paper describes the preparation and characterization of

self-assembled monolayers (SAMs) formed by the chemisorptionof alkanethiols (HS(CHz),R) on surfaces of copper and silver.Both of these metal surfaces are highly active in the chemisorptionof organosulfur compounds.H They offer an excellent opportunitlto study the influences of the substrate in def rning and controllingthe order of self-assembled monolayer films. A central objectiveof this study was to compare the structures of the monolayersformed on silver and copper with those formed by chemisorptionof alkanethiols on Bold.r-rr The molecular orientation and or-dering of long-chain alkyl thiolate monolayers on gold reflectsthe epitaxial bonding of the sulfur atoms (or, perhaps moreprecisely, of gold thiolates) to the surface lattice plane. Forlong-chain alkyl disulfides and thiols chemisorbed on Au( I l I )(the preferred crystallographic texture of typical polycrystallinesamples), electron diffraction (TEM and LEED)r0 has establishedthat the sulfur atoms in the adsorbates form (V3 x y'3)R30'werlaycn. This epitaxialoverlayer results in an interchain spacingof 4.99 A and is well suited to the formation of close-packed, andtherefore ordered, organic surface phases when the alkyl chainis long (n > 15) and the R group roughly commensurate in sizewith the polymethylene chain.ll'12 Several recent studies discussthese issues.?-13

Copper, silver, and gold have nearest neighbor spacings in the( l l l ) p lane o f 2 .56 A, 2 .89 A, and 2 .88 A, respec t ive ly . ra I fstructures form on copper and silver that are analogous to thoseon gold (i.e., (y'3 x /3)R30o), we expect the molecular orien-

t Harvard University.I Pcnnsylvania State Univcrsity.I Pcrmanent Address: Institutc of Chemistry, Academia Sinica, Taipei,

Taiwan, ROC.# AT&T Bcll Laboratories; corrcspondcncc should bc addrcssed to De-

partmcnt of Matcrials Science and Engineering, University of lllinois atUrbana-Champaign, Urbana, IL 61801.

CopyrightReprinted from the Journal of the American Chemical Society, 1991, l. lJ.

O 1991 by the American Chemical Society and reprinted by permission of the copyright owner.

Comparison of the Structures and Wetting Properties ofSelf-Assembled Monolayers of n-Alkanethiols on the CoinageMetal Surfaces, Cu, Ag, Aul

Paul E. Laibinis,t'2 George M. Whitesides,*'t David L. Allara.*'t Yu-Tai Tao,l'$Atul N. Parikh,l and Ralph G. Nuzzo*'#

Contributionfrom the Department of Chemisny, Haruard Uniuerstty,Cambridge, Massachusetts 02l,38, Departments of Materials Science and Chemisty,Pennsyluania State Uniuersity, Uniuersity Park, Pennsylt',ania 16802, Departments of MaterialsScience and Chemistry, Uniuersity of Illinois at Urbana-Champaign, Urbana, Illinois 6/,80/,, andAT&T Bell Laboratories, Murrav Hil l, New Jerser'07974. Receiued January 14, I99l

Abstrect Long-chain alkanethiols, HS(CH2)nCHl, adsorb from solution onto the surfaces of gold, silver, and copper and formmonolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared)have the chains in welldefined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayerson silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayersformed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation.

Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that thealkyl chains in monolayers on si lver are al l- trans zig-zag and canted by - l2o from the normal to the surface. The analysisalso suggests a twist of the plane containing the carbon backbone of -45o from the plane defined by the tilt and surface normalvectors. For comparison, the monolayers that form on adsorption of alkanethiols cn gold surfaces, as judged by their vibrationalspect ra , are a lso t rans z ig-zag extended but . when in terpreted in the conter t o f the same srng le cha in model . have a cant angleof -2'7o and a twist of the plane of the carbon backbone of -53o The monolarcrs iormed on copper (when they are obtainedin h igh qual i ty ) exh ib i t in f rared spect ra e f iec t ive ly ind is t ingurshable i ronr thosc on: r l ie r and thus appear to have the samestructure. Fi lms on @pper are also commonll obtained that are structurai i \ i l l -det ' ined and appear to contain signif icant densit iesof gaucheconformations. Thesespectroscopical ly 'based interpretat ions are compatible *rth inferences from wett ing and XPSmeasurements. The structure of the substrate-sulfur interface appears to control molecular orientat ions of the alkyl groupsin these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependentpopulation of gauche conformations, is presented and applied to the specific case of the structures formed on gold.

tations of the chains to differ significantly since the average chaindensity on copper would have to be considerably higher than ongold or si lver ( - 17.03 A2Tchain on copper versus 21.75 l*2lchainon s i lver and 11.62 A2, tcha in on go ld) . We would expect , g iven

( l ) Thrs hrghlr col laboratrve work was supported by a var iety of sources.G.M W acknowledges support f rom the Oi f ice of Naval Research, the De-fense Advanced Research Pro. lects Agency (DARPA), anci the Nat ionalScience Foundat ion (Grant CHE-88-12109). The XPS spectrometer wasobtained through the DARPA University Research Initiative and maintainedin the Harvard Materials Research Laboratory. D.L.A. and A.N.P. ac-knowledge support in part from the National Science Foundation (GrantDMR-9001270) .

(2) Shell Foundation Pre-doctoral Fellow, 1987-88.(3) Seymour, D. L.; Bao, S.; McConville, C. F.;Crapper, M. D.;Woodruff,

D. P. ; Jones, R. G. Suy' Scr. 1987, 189-190,529-534. Anderson, S. E. ;Nyberg, C. L. J. Electron Spectrosc. Relat. Phenom. 19X1,52,135-'146.

(4) Sandroff, C. J.; Herschbach, D. R. /. Phys. Chem. 19t2, 86,3277-3279.

(5) Sandroff, C. J.l Garoff, S.; Leung, K. P. Chem. Phys. ktt.l9t3, 96,547-551. Joo, T. H. ; Kim, K. ; Kim, M. S. " / . Phys. Chem. 19t5,90,58 l6-58 19. Sobocinski, R. L,; Bryant, M. A.; Pemberton, J. E. "/. Am. Chem.Soc. 1990. I 12, 6l'17-6183.

(6) Kwon, C. K.; Kim, K.: Kim, M. S.; Lee, S. B. 8rll, Korean Chem. Scr.19t9. /0. 254-258.

(7) Nuzzo, R. G.l Allara, D. L. J, Am. Chem. Soc. 1983, 105,4481-4483.(8) Bain, C. D. ;Troughton, E. B. ;Tao, Y.-T. ; Eval l , J . ; Whi tesides, G. M.;

Nuzzo, R. G. / . Am. Chem. Soc. 1989, 1 l l ,32l-335.(9) Porter , M. D. ; Br ight , T. B, ; Al lara, D. L. ; Chidsey, C. E. D. J. Am.

Chem. Soc. 19t7, i09,3559-3568,(10) TEM: Strong, L, ; Whi tesides, G. M. Langmuir l9t t , 4, 546-558.

LEED: Dubois, L. H. ; Nuzzo, R. G. Unpubl ished resul ts.( I I ) Chidsey, C. E. D.t Liu, G.-Y.; Rowntrec, P.; Scoles, G. J. Chem. Phys.

19t9, 9/ , 4421-4423. Cami l lone, N, , I I I ; Chidsey, C. E. D. ; L iu, G.-Y. ;Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991,94,8493-8502.

(12) Nuzzo, R. G.;Dubois, L. H. ;Al lara, D. L, / . Am. Chem. Soc. 1990,t t 2, 558-564.

(13) Whitesides, G. M.; Laib in is,P.E. Langmuir 19fr ,6, 87-96. Bain,C. D. ; Whi tesides, G. M. Angew. Chem,, lnt . Ed. Engl .1989, I0 l ,522-528.

(14) Ki t te l , C. Sol id State Physics, 5th ed. t Wi ley: New York, 1976; p32.

a \ t O O I A - ^ - : ^ ^ - / - L ^ - : ^ ^ | ( f ^ ^ i ^ r , ,n n n a t o < 1 1 ( ) 1 l 1 < 1 2 t r ( t o n a < n / n

Monola.yers of n-Alkanethiols on Cu, Ag, and Au Surfaces

this constraint, that the chains on copper would be aligned closerto the surface normal direction than those on silver or gold. Theinterchain spacing in the orthorhombic form of polyethylene yieldsan average chain density in the ab plane of - 18.27 A2lchain.r5The epi taxial bonding of a lkyl th io lates in a (V3 x V3)R30"overlayer on copper therefore may be precluded by the stericpacking constraints imposed by the chains. Silver and gold, onthe other hand. have very similar lattice constants, and we expectthe monolayers formed on them to have similar structures in theabsence of other perturbalions.

The studies described here demonstrate that the structuresformed by similar adsorbates on polycrystalline copper, silver. andgold surfaces with a preferred ( I I I ) texture are related butdifferent. The structural characteristics of monolayers formedon copper are extremely sensitive to the history of the samplesand the details of the experimental techniques used in theirpreparations. The best films we have obtained on copper werecomprised of closest packed chains that contained few gaucheconformations. The poorer quality films contained significantcnncentrations of gauche conformations. The monolayers obtainedon silver are highly organized and contain only low concentrationsof gauche conformations. The molecular orientations deducedfrom the data are compatible with surface phases for alkane-thiolates on silver and copper that are close-packed and havesrgnificantly shorter interchain spacings than that found on gold.

We present evidence, both from our studies and from the lit-erature, that the differences in the structures of the monolayersfbrmed on copper, silver, and gold reflect the differences in thereactivity of the surface of the metal. particularly its susceptibilityto oxidation on exposure to laboratory air. The surfaces of copperoxidize rapidly l6 (and may also adsorb other environmentalcontaminants) dur ing preparat ion of the sample. Si lver alsooxidizes on exflosure to the laboratory atmosphere. but oridationis substant ia l l i less extensi ' , 'e than for copper: under ambientconditions, one or two monolavers of silver oxide form on the silversur face . r ' The adsorp t ion o f a lkaneth io ls on s i l ver i s complexand involves. we believe. some combination of reduction of Ag(l)to Ag(0), direct conversion of Ag(l) surface oxides to Ag(I)thiolates, and formation of an ionic surface phase in which silveratoms have been removed from their equilibrium lattice positions.We use precedents established by LEED studies to derive thismodel and to rationalize the orientations determined by infraredsp€ctroscopy. The data also suggest that, on prolonged exposureto the adsorbate solution, the surface becomes extensively sulfided,presumably forming a thin interfacial layer of Ag2S.rt Gold doesnot oxidize under ambient conditions. and the mechanism ofconversion of alkanethiols to gold(l) alkanethiolates has sti l l notbeen established unequivocally.re The high activity of silver andcopper to oxygen and to thiols established the assembly procedureswe used for reproducible formation of high-quality monolayers.The length of exposure of the unfunctionalized surface to theatmosphere, the solubil ity of the adsorbate in the contacringsolvent, and the t ime of exposure to the solut ion of a lkanethiol--variables that are generally unimportant in the reproducibleassembiy of alkanethiols on gold-were very important on copperand moderately important on silver.

ResultsPhenomenological Differences in the Assembly of Alkanethiols

on Copper, Silver, and Gold. The assembly of high-quality al-kanethiolate monolayers on gold is straightforward.T-13'20'21 Silver

J . Am. Chem. Soc . , ' , ' a l . I l 3 , No . 19 , I 99 l 7153

and copper, in contrast. readi lv rorm oxides that adsorb polarcontaminants. We suspect the surfaces of these metals becornerougher and more heterogeneous * ' i th ertended exposure to air.Oxidized surfaces of copper and sih'er are acrive to the chemi-sorption of alkanethiols; the monola.vers formed dif fer. howsysr,in structure and propert ies from those formed on less oridizeCsur faces. The d i f ferences can be ra t iona l ized br hrpothcs izrngthat the monolayer has formed on a roughened metal meral c''rrdeinterphase and may itself be structurally heterogeneous The mostreproduc ib le and h ighest qual i ty monolayers *e hare obrarncdon si lver and copper resulted from procedures in uhrch ireshlyevaporated films were transferred rapidly under inert armtxpheresin to adsorbate-conta in ing so lu t ions, Manipu lar ion o i unfunc-t iona l ized subst ra tes in a i r , pr ior to der ivat izat ion. resu l ted inmonolayers that exhibited larger hystereses in contact angles andappeared less crys ta l l ine by IR and composi t iona l lv less homo-geneous by XPS.

We begin by comment ing on the in f luence of the so l rent onthe monolayers formed from thiols dissolved in them.:: \ leexposed sl ides of the three metals to I mM octadecanethiol so-lut ions in a range of solvents ( isooctane. chloroform. tetra-hydrofuran, acetonitr i le, acetone, and ethano!) for 2 to 4 h andmeasured the advancing and receding contact angles (0^ and 0,)of water and hexadecane (HD). The slides were transferred fromthe evaporator into solut ions under f lowing argon to minimizeeffects due to laboratory exposure. In general, we observed nosignificant differences in wetting (or hysteresis = cos d, - cos 0r)that could be attributed to the solvent (wetting data are availablein the supplementary material). Monolayers formed on copperexhibited greater hvsteresis than those formed on gold or si lver.The wet tab i l r t ic . o f - thc nonola l 'e rs formed on a l l three meta lsare stmil , i r t t) thr-xc rr 'pxrrted for oriented alkvl monolayers on othersubs t ra tes ' l ' r ' \PS da ta a l so demons t ra te t ha t t he so i ven tsl i s t ed abo r c r r s r r n r c i c t i \ e t o r i a rd t he subs t ra tes unde r t hesecond i t i ons .

Copprer and si lver did prove to be more reactive to alkanethiolsand more sensit ive to attr ibutes of the alkanethiol than gold. Forexample, react ion o f shor t a lkaneth io ls (n < 3) wi th the copperand si lver surfaces often resulted in surfaces that appeared dis-colored and sometimes visibly pitted; continued exposure seemedto result in dissolution of the metal. With increased chain length,the reproducibi l i ty of formation of high-quali ty monolayers (asdetermined by wetting and XPS) increased dramatically.2T Withlonger alkanethiols. the solubi l i ty of the adsorbate became im-portant. Dcrcosanethiol. which is soluble in ethanol with warming,often formed mult i layered structures on copper when adsorbedf rom ethanol :28 adsorpt icn f rom isooctane reproduc ib lv resu l tedin n rono le re r f i lms

{dso rp t i on t i n res o f I t o l 2 h y re lded mono laye rs w i t h t heg rea tes t r ep roduc ib i l i t r ; ue used t hese i n te r va l s i n t he s tud ies

(20) Barn, C, D. ; Eval l , J . ; Whi tesides, G. M. J. Am. Chem. Soc. 19t9.i l t , 7 t 5 5 - 7 t 6 4 .

(21) Laib in is, P. E. l Hickman. J. J . ; Wrighton, M. S. ; Whi tesides, G. M.Science (llashington, DC) 1989, 245,845-847.

(22) The nature of the solvent used has been shown to affect the wettabilityof monolayers on gold derived from polar-terminated alkanethiols.m Extensionof these systems to the generation of high-energy organic interfaces requiresthe assembly be compatible with polar, hydrogen-bonding solvents.

(23) Cohen, S. R. ; Naaman, R.; Sagiv, J. J . Phys. Chem. 1986,90,3054-3056. Ti l lman, N. ; Ulman, A. ;Schi ldkraut , J . S. ; Penner, T. C. J. Am.Chem..Soc. l9tt, I 10,6136-4144. Wasserman, S. R.; Tao, Y.-T.; Whitesides,G. M. Langmuir 19t9, J, 1074-1087.

(24) Allara, D. L.; Nuzzo, R. G. Iangmuir 1985, /, 45-52.(25) Chen, S. H.; Frank, C. W. I"angmuir 19t9, J, 978-987.(26) Troughton, E. B. ; Bain, C. D. ; Whi tesides, G. M.; Nuzzo, R, G,;

Al lara, D. L. ; Porter , M, D. Langmuir t988, 4, 365-385.(27) We believe that the alkane provides a passivating layer that slows

corrosion of the copper and silver by the alkanethiols.(28) The fi lms formed from ethanol were extremely hydrophobic 0fro -

l30o), exhibited large hystereses, and were wet by hexadecane. We bclievcthe wetting behavior to be consistent with formation of a porous, precipitatedlayer of alkyl thiol andfor disulfide. The fi lms prepared in ethanol could bcgrown to the point of being visible and were robust to washing with ethanol;washing with hexanes resulted in dissolution but did not reveal an underlyingmonolayer film that was not wet by hexadecane, This problem was more acuteon copper than on silver; it was also more prevalent at lower temperatures.

(15) Bunn, C.W. Trans, Faraday Soc. 1939, 35,482-491. Walter, E. R.;Rcding, F. P. / . Polym, Sci . t956, 2t ,561-562.

(16 ) Law less , K . R . ; Gwathmey , A .T . Ac ta Meta l l . 1956 ,4 ,153-163 .(17) Rovida, G.; Pratesi , F. ; Magl iet ta, M.; Ferroni ,E, Surf . Sci , 1974,

43,23U256. Rovida, G,; Pratesi, F. Surf. Scd. 1975, 52,542:5SS, Engle-hardt, H. A.; Menzel, D. Surf..Sci. 1976, 57,591-618 and references citedtherein.

(l.q) The promotion of silver tarnishing by sulfur-containing compoundsis wel l known. See, for example: Gracdel ,T. E. ; Franey, J.p, ;baul t ier i , G.J . ; Kammlo t t , K , W. ; Ma lm, D .L .Conos . .Sc j . l g t5 ; 2J , l t 63* l lE0andreferences cited therein.

_ ( !q l An oxid iz ing coreactant is not required: Dubois, L. H. : Nuzzo, R.O. Unpubl ished resulrs,

s38 53' 528 523 173 165 155

(tI

o

xo

P

cJoo

.=oco+t

5

I99 I

201 00

7060504030201 00

7060504030201 00

1000 800 600 400 200

Binding Energy (eV)Figure t. XPS spectra of monolayers of octadecanethiol adsorbed onsurfaces of copper, silver, and gold. High-resolution spectra of the O( I s)and S(2p) regions are presented in the insets and were referenced toC( I s) = 284.80 eV. The dashed lines are provided for comparison of theS(2p) spectra. The binding energies of S(2s) and S(2p372) are given inTable I and compared with values obtained for docosanethiol.

whose descriptions follow. Greater reproducibility on silver andcopper was obtained with longer n-alkanethiols when isooctanewas used as the solvent owing to the increased solubi l i t l ' of theadsorbates studied in this solvent, Adsorptions onto coppersurfaces were conducted from isooctane; adsorptions onto thesurfaca of gold and silver used ethanol or isooctane, No structuraldifferences in the high-quality films were observed on any metalthat could be assigned to solvent; those differences that wereobserved, we attribute to the solubility of the adsorbate or tosurface roughening.

As indicated, to minimize the effects of oxidation and con-tamination, we transferred the slides of copper and silver to ni-trogen-purged solutions of the thiols under a flowing stream ofargon immediately following the evaporation of the metal. Oc-casionally, this procedure did not result in formation of high-quality monolayers on copper. We believe, however, that in-strumental parameters for the evaporator (background pressureduring the deposition, argon flow rate, etc.) were responsible forthese failures and stress the importance of excluding oxygen (tothe extent possiblc) to form monolayers on copper of the qualitywe have characterized. Silver could be exposed to air, to a limiteddegree, without exhibit ing the catastrophic failures found forcopper. We decided, based on the insensitivity of gold to theatmosphere, that anaerobic transfer was unnecessary; the filmswere manipulated in air and transferred to solution within I hof evaporation.

The monolayers formed on all three metals are stable to physicalmanipulation and washing with polar solvents. The monolayerson copper and silver do, however, exhibit only l imited stabil ityto air (changes are observed after - I week of exposure),2e but

Laibinis et al

Table L Wetting Properties and XPS Binding Energies ofMonolayers of Octadecanethiol Adsorbed on Copper, Silver, andGold and of Related Materials

XPS bindingwetting (0", 0r),o energies (eV)D

Hzo HD S(2p:iz) S(2s)

HSC22H4sAurSC,5HrTCu/SC1sH37Ag/SC1sH37A u / S C 1 6 H 3 7Ag.Ag2O + HOrSCr6H33

NA NANA NA120 , t 03 48 ,32I 16, 103 50, 40I 15 , 105 48 , 369 6 , 6 l < 1 5 , - '

t63.6 . 217.9162.6dt62.t 226.2t62.3 225.7l6l .9 226.4167 .4 231.6

o HD = hexadecane. NA = not applicable. b Binding energies for allsamples were referenced to C(ts; = 284.80 eV. 'For comparison, ref3l reports a value of 163.3 eV (referenced to C(ts; = 284,7 eY).dReference 31. Gold(l) thiolates are polymeric species;caution shouldbe exercised in comparing their spectral properties directly with thoseobtained on monolayers. 'A dash indicates a receding constant anglefor a contacting liquid that could not be removed from the surface; d, ::0 0 .

could be routinely manipulated in ways previously detailed foralkanethiolate monolayers assembled on gold.E

Scanning Electron Microscopy (SEM). The morphologies ofthe various substrates after exposure to octadecanethiol wereexamined by scanning electron microscopy. The surfaces of thethree substrates. when preoared under comparable conditions, aresimi lar . The copper and si lver samples appear to have the same'rol l ing hr l ls- tcr ture that has been descr ibed for f i lms of goldevaporated onto sr l icon wafers.s The amount of roughening dueto oxidat ion or react ion of the copper or s i lver surfaces withalkanethiols does not appear to induce any perceptible morpho-logical changes at the level of resolution of SEM ( -20f500 A).

X-ray Photoelectron Spectroscopy (XPS). Representative XPSspectra of monolayers obtained by reaction of octadecanethiol oncopper, silver, and gold are given in Figure l. The most significantfeature in these spectra is the content of impurities of the films,as inferred from the intensities of peaks in the O(ls) core levelregion (binding energy -535 eV). Clearly, the alkanethiolatemonolayer on copper is adsorbed on an interphase containing asignificant quantity crf oxide. Treatment of highly oxidized coppersurfaces with th io ls resul ts, as judged by XPS, in a decrease inthe content of ox1'gen and a complete loss of peaks due to Cu(tl).DThe residual oxlgen content of the copp€r interface remains high,however, suggest ing that only a part ia l d isplacement of near-surface oxlgen atoms occurs on exposure to the adsorbate. Theresidual oxide content of the monolayer on silver is very low,perhaps of the order of a few percent of a monolayer. The ad-sorption of thiol on silver thus must either reverse any oxidationof the substrate that has occurred or displace a significant fractionof oxygen species generated by it. Further discussion of this pointwill be deferred to the Discussion section. On gold, the oxygencontent of the film is below the detection limit of XPS. Thecharacter of the sulfur-surface bond is indicated by the high-resolution core level data in the S(2p) region (Table I and Figurel). The binding energies measured for the 2p312core level oncopper, silver, and gold ( I 62. I , 162.3, and I 6l .9 eV, respectively)are all well within the range expected for a surface thiolate (RS-Mor RS-M+) species.3o'3r These values would implicate a formaldissociative adsorption of the S-H bond on the metal on all threesubstrates (eq 1;.1'o'lt

RSH + M(0) -* RS-M(I) + "Y2H2"

RSH + M( l ) * RS-M( | ) + *H+ '

( l a )

( l b )

We can define the metal-sulfur bonding in more detail by ex-amining the l ine shapes of the S(2p) core levels. In carrying out

(30) Nuzzo, R. G.; Zcgarski, B. R.; Dubois, L. H. J. Am. Chem. Soc.t9t7 . t 09 ,

't 33-1 4A.(31) Bain, C. D,; Biebuyck, H, A,; Whitesides, G, M. Langmuir 19t9, J,

723-727.

As/S(cHJ,rcH. ' i ]ot"r

I E1 '3

: l lF l :zl-*^^"*-l ,l zo] .__ ' - - - - ] l l o

r75 165 1ss

(29) Laibinis, P. E.; Whitesides, C. M. Unpublished results.

r r l l r r l t i r ; l r r i l r i l 1

ogoooooE ssoE!8 6

.c![33'i !]:AS;

Monolayers of n-Alkanethiols on Cu, Ag, and Au Surfaces

I 20.

90"

600

120

900

600

120.

90"

600

nFigure 2. Wetting of monolayers of n-alkanethiols (CHr(CHr)SH)adsorbed on copper (upper), silver (center), and gold (lower) by *ater(squares) and hexadecane (circles). Advancing and receding angles arefilled and open symbols, respectively. F:ch point represents the averagevalue obtained on a slide (see Experimental Section). Exposure of copperand silver to the atmosphere was minimized prior to immersionl gold wastransferred in air to adsorbate solution within I h of exposure to theatmosphere. Silvcr and gold were exposed to I mM solutions of al-kanethiol in ethanolor isooctane for 4-12 h at room temperature underargon to form the monolayer; rinsed with ethanol, blown dry with N2,and characterized; monolayers on copper were adsorbed from isooctaneusing similar procedures.

this examination, it is necessary to use data from analyses in whichthe X-ray exposure has been minimized. The core-level spectraindicate that the alkanethiolate monolayers degrade on prolongedexposure to X-rays. The spectra on silver and gold can be fitreasonably well, even given the noise in the spectra. as twospin-orbit-spin components of equal width (1.0 eV) whose inte-grated areas are in the rat io 2J + | ( i .e. , I :2; J = | 12,31). Thespinrrbit splitt ing used (1.15 eV) was taken from the l iterature.32The spectra for samples on copp€r occasionally could also be fitwithin these constraints. Spectra were routinely obtained wherethe l ine widths were s igni f icant ly greater (1.2-1.4 eV) and theresolution between the two components significantly poorer thanthose obtained on gold or silver samples. There is no unambiguousway to interpret these larger line widths; we suspect, however, thatthey reflect compositional heterogeneity.

Wetting Properties. Static advancing and receding contactangles of water and hexadecane (HD) were measured on mono-layers prepared by the immersion of substrates in solutions ofn-alkanethiols of various length (Figure 2). The wetting behaviorson the three metals are similar, with the contact angles of bothliquids being significantly lower when n < 8 than when n 2 l l .Porter et al. have shown previously for monolayers formed on goldthat the inferred degree of crystallinity of monolayers changesin this region.e We defer further discussion of the relations be-tween wetting and structural features to later in the manuscript.The observed hysteresis (A cos d = cos 0, - cos d.) is smallest formonolayers formed on gold and slightly larger on silver and copper.This increased hysteresis may reflect an underlying difference in

J. Am. Chem. Soc. , Vo l . I13, No. 19, I99 l 7155

the roughness of the two surfaces that is not detectable by SEM.In general terms, however, the wetting measurements suggest thatthe monolayers are all oriented, and that the.v- expose a low-energymethyl surfaces'23-26 and. from the perspective of the outermostfew Angstroms, appear to be similar.

Investigation of the Thickness of the Monohyer Films by El-lipsometry and XPS. l. Ellipsometry. Ellipsometr! is an inva'luable technique for measuring i i lm thicknesses of alkanethiolsadsorbed on gold.7-e We attempted to measure the change inthickness on samples of'copper and silver that had been exposedto air for only short intervals ((15 min) prior to exposure to analkanethiol solut ion. We could, with considerable effort (see

Experimental Section), obtain thicknesses on silver that could berelated to the length of the adsorbate; ralues *el l outside theexpected range of thickness were, however. also routinelv obuined.Given the lack of reproducibi l i ty in our measureme nts. we arehesistant to make any structural conclusions based on relat ingel l ipsometric thickness to the chain length of the adsorbate'Acceptable thicknesses could not be obtained on copp€r.

2. XPS Attenuation. XPS offers an independent method tocompare thicknesses of monolayers prepared by direct exposureof the metal to an alkanethiol. In contrast with el l ipsometry,characterization of the unfunctionalized interface is not required'The determination of the absolute thickness of a *thin' layer byXPS requires, at a minimum, precise values of escape depths and

atomic cross sections, parameters difficult to obtain with therequired accuracy. XPS does provide, however, a convenient andrapid method for comparing differences in relative thicknessesover a homologous series such as that represented by the n-al-kanethiols. The logarithm of the intensity, /y, of photoelectronsf rom a semi in f in i te layer (>200 A) a t tenuated by an n-a lky lorer la ler is propor t iona l to - (nd l \ , where d is the th ickness ofthe monolaler per methl lene group, n is the number of methylenegroups in each chain , and ) , is the a t tenuat ion length o f thephotoeiectrons from the underlying substrate through hydro-carbons.33 A plot of ln ( intensity) versus n provides both aqualitative assessment of the regularity of the thickness of mon-olayers prepared from different length alkanethiols and a com-parison between monolayers formed on different metals. Becausethe escape depth (I) is also a function of the kinetic energy ofthe photoelectron,3a we selected specific core level peaks basedon both their cross section and proximity to peak of other metals.

The intensity of photoelectrons arising from the underlyingsubstrate of kinetic energies of - I I 25 eV (Au(4drrr) = I 134 eV,A g ( 3 d 5 z ) = 1 1 1 9 e V ) a n d - 1 4 0 0 e V ( A u ( 4 f 7 , 2 ) = 1 4 0 2 e V ,Cu (3p ) = l 4 l 2 e \ ' ; a re p l o t t ed i n F igu re 3 f o r mono laye rs p re -pared from n-alkanethiols of different lengths.3s These plos followthe expected Iogar i thmrc re la t ionsh ip . The in tens i ty o f photo-

e lect rons for monola lers on s i lver and copper a t tenuate morerapidly (by - 20Vc) than for monola)'ers on gold. This differencesuggests that the films formed on copper and silver are each -20Vo

thicker than those formed on gold, This observation is consistentwith the differences in molecular orientation infened from infraredspectroscopy (see below).

Investigation of Film Structure and Molecular Orientrtion byIR Vibrational Spectroocopy. l. Obsened Spectra of MonolayersIR spectra of the various monolayers were obtained using a singlereflection geometry (-85o angle of incidence) as described inthe ExperimentalSection using ppolarized light. In earlier papers,quantitative analyses of such data have been presented formonolayers of various substituted alkanethiols and dialkyl di-sulfides on gold.r2'36 The present study seeks to present aquantitative correlation of thin-film structure as exhibited in ahomologous series of adsorbates across these related substrates.To facilitate these analyses, the previous computational proto-

(33) Bain, C. D.; Whitesides, G. M, J. Phys. Chem.19t9, 93, 167f 1673'(34) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, /,2-ll.(35) A similar slope was obtained for alkanethiols (C5-C1s) adsorbcd on

copper from ethanol even though the monolayers exhibited significant contactangle hysteresis,

(36) Nuzzo, R. G.; Fusco, F. A.; Allara, D, L. J. Am, Chem. Soc. 19t7,t09.2358-2367.

-0.5

1 . 0

-0.5

e\r/ 0.0ooo 0.5

3000o

30"0o

30'0o1 . 0

1 .0

-0.5

..5:;il:5 ';:F:.;

o a r s g o oo9ogoo

- ; [ r [ l t l l l t

lioooaoEo;84il:E:

r"a8ffi368 Ss6:.gg6 g

7156 J. Am. Chem. Soc., Vol. II3, No. 19, l99l

Teble II. Description of Vibrational Modes of C-H Stretching for Polycrystalline Alkyl-S Chains C16H33S - C22H45S,

Laibinis et al.

mode descriptionD peak frequency (cm-r)'assigned direction of transition dipole

moment matrix element in molecular coordinates

CH3 asym ( ip) , ( r " - )CH3 asym (op), (ru-)CH3 sym, r+ (FRC)CH2 asym, d- (a)CH2 asym, d-CH2 sym, d+ (FRC)CH3 sym, r+CH2 sym, d* (o)CH2 sym, d*

296429542935292529182895-2907287928532850

ip of CCC backbone, I C-CH3 bondl- CCC backbone planell to C-CH: bondl- CCC backbone plane-L CCC backbone planeip CCC backbone plane, ip HCH planell to C-CHr bondip CCC backbone plane, ip HCH planeip CCC backbone plane, ip HCH plane

o B.scd on data takcn for [C,H2r rS] 2 samples used for optical function determinations. 'Abbrcviations uscd: asym = asymmctric or antisym.mctdc, sym = symmctric, ip = in planc, op = out of plane. FRC = Fermi r€sonance splitting component. a = CHr groups located at cithcr end ofthe alkyl chain, ll = psr.llcl, l- = p.rpcndicular. 'Pcak frequency is taken as that from the imaginary optical function (t) spectrum. Peak frcqucncyvalues fot modcs of intcnsitics too wcak to measurc directly havc been assigned using average values taken from previously compiled rssignments (rcf9, 12,36,37), Thesc values werc suboequently used in the fittin8 routines used to generate the components ofthe simulated spectra (sec supplc-mcntarY material).

nFigre 3, Intensity of photoelectrons due to the underlying subetrates formonolayers on gold, silver, and coppcr formed after I h exposure to ImM solut ions of n-alkanethiols (CH3(CH2)/SH), The upper f igureprovides comparison of monolayers formed on gold and silver, the loweron gold and coppcr. The points represent single measurements. The datasets have been offset vertically to enable comparison; the slope is theparameter that is important here. The kinetic energies of the photo-clectrons in each graph are similar: (upper) Au(4d372) = I 134 eV,Ag(3d57) = I l l9 eV; (lower) Au(4f772) = 1402 eV; Cu(3p) -- l4l2 eY.Lines were obtained by least-squares analysis and gave the followingslopes (see text): Au(4d372), -0.021: Ag(3drrr), -0.025; Au(4f772),{.020: Cu(3p), 4.024.

colslll6'37 were modified (see below). Further, since we anticipatodthat the differences between specific adsorbates of interest to usmight bc small, our earlier work was reexamined with the intentionof minimizing expcrimental errors that complicate a precisequantitative comparison of the data obtained from differentsubstrate materials. The analyses that follow are based on multiplesample preparations conducted in threc different laboratories andspcctral data acquired on two different instruments. The largestvariations we observed for comparable instrument optics arecompatible with the stat ist ical variat ion inherent in mult iplesamples. The error bars appearing in the figures reflect the scatterdominated by effects duc to indepcndent measurement of multiplesamples. lt is important to note that the data shown below reflect

our specific optical design. Large differences in absolute spectralintensity will result if even slightly different beam divergencesand angles of incidence are employed.ss

The infrared data for the series of monolayers on copper, silver,and gold are shown in Figure 4. The data shown for copper arefor the more well-defined structures we have obtained (a repre-sentative example of spectra obtained for ill-defined monolayerson copper is given in the supplementary material). The sets ofdata reveal substantial vanations in the C-H vibrat ional modeintens i t ies as a funct ion o f cha in length for go ld as compared tothe other two : \s tems of monolayers (copper , s i lver ) . Given theclose similari t i ol the copper data to that of si lver, we wil l restr ictour discussion to the better-behaved silver system (although similardiscussions would pertain for this specif ic copper data set). Wewill discuss for gold the CH3 mode intensities first and follow withan examination of those of the methylenes. The basis of theassignment of the spectral features to specific vibrational modeshas been reported previouslyl2'36'37 and will not be repeated here.Details of these fairly complex fittings and assignments are givenin the supplementary material. Table II presents exact descriptionsof the assignments, the approximate position of the band, as wellas transition dipole directions for the mode as expressed in mo-lecular coordinates. The latter information wil l be used forcalculat ions of structure discussed in the next section. Ourquanti tat ive interpretat ions of molecular orientat ions wil l be re-str icted to analy'ses based on modes appearing in this high-fre-quency region. The lower frequency modes, while r ich in infor-mation (ser below), are sufficiently weak in intensity as to precludequant i ta t ive analys is .

The strongest of the CH3 modes in this region of the spectrumis the symmetric methyl stretch (r+), which is spl i t by Fermiresonance into two separate bands appearing at -2878 and-2935 cm-r, and the in-plane asymmetric stretch (r.-) appearingat2964 cm-r. The out-of-plane asymmetric stretch (16-), whichoccurs prominently at 2954 cm-r in the polycrystalline bulk phasespectra, is weak in the monolayer spectra; this mode is a weakshoulder on the r"- band. The line shapes do not appear to varysignificantly across this series of monolayers, allowing band in-tensities to be estimated for most bands (ru- excepted) by theintensity at the peak maximum. Figure 5 shows the intensity dataof the r+ (2877 cm-r) and rr- (2963 cff i-r , contains a small con-tribution from 16- absorption) bands for the Cru-Czo films on goldand silver. On gold, the data are given normalized relative to thevalue of the intensity of the selected mode for the C16 film. Thetheoretical plots shown for gold of the modulation of these de-viations in intensity are based solely on the changes in geometryof the CH3 group predicted for two possible models of chain

(3E) In particular, the angle of incidence must be correctly set within lessthan lo, thc//number of the beam must be fixed at a high value, -//15 -

f 120 for our two instruments, and stray light must bc excluded from collectionof the reflected beam off the sample. We (D.L.A., R.G.N.) observcd fluc-tuations as much as a factor of 2 bctween misaligncd, fast optics and thcconfiguration above. Additional deviations could also involve dctector non-linearities for high throughput configurations.

65

4

F*rO o! -O x- c 9 1

tr

K,3 6x - 5

4

201 51 0 25

,/a AuglTpl

v - - -V.. l r

l-..-. --t-

( \ot r'

cu(3p) a)t

(37) Allara, D. L.; Nuzzo, R. G, Langmuir 1985, /, 52-66,

Monolayers of n-Alkanethiols on Cu, Ag,

structure and not on changes in the vibrational states. Thesemodels will be discussed in detail later in the manuscript. Theexperimentally observed fluctuations in odd-ven intensities arestriking. It is also obvious that this experimental behavior presentsan important criterion for determining the validity of variousstructural models. In an earlier publication,l2 these variationswere noted qualitatively and used to assign a constant f ixed di-rection of chain ti l t with a consequent odd-even dependence ofthe orientation of the C-CH3 bond with respect to the normalto the surface. This point wil l be discussed in detail later in themanuscript.

Figure 5 also shows that the r+ and rr- band intensities (ab-solute) on silver exhibit, to within experimental error, no significantmodulation with chain length. This difference between the spectraobtained on silver (and similarly on copper) and on gold stronglysuggests a difference in structure of monolayers on these supports;those on silver (and copper) indicate that the projection of theC-CH3 bond on the normal to the surface is invariant with chainlength.

The C-H stretching mode intensities (absolute) of the CH2group d* and d- modes exhibit a linear dependence on chain lengthfor monolayers supported on gold and silver (plots are given inthe supplementary materials).3e No odd-even variations areevident, suggesting, within the experimental uncertainty of themeasurement, that there exists a constant chain orientation onthe surface for these films, one that differs in magnitude for silverand gold.

2, Iletermination of Optical Functions. Descriptions of themethods used for determining precise optical functions, an overviewof relevant error analysis, and representative data used in thetheoretical modell ing are given in the supplementary materials.

3. Calcuhtion of Averrge Molecular Orientrtion in MonolayerFilms on Copper, Silver, rnd Gold. The theoretical basis forderiving an average orientation of molecules in monolayers fromIR spectra of metal supported films has been presented in detailelsewhere.3T As presented then, the essentiat operations involvedwere ( I ) the determination of a set of isotropic optical functionsapplicable to the monolayer films;{ (2) the experimental deter-

(39) Linear least-squares fits to thc intensitics of the d+ and d- modesvcrsus chain length for monolayen supportcd on gold and silvcr intcrsect zcrointcnsity at chain lcngths significantly greater than zero, Plots and furtherdiscussion are includcd in thc supplcmcntary matcrials,

t 13 , No. j ,9 , I99 l 7157

r l l

2800I '

3000

mination of the thickness of the frlm from ellipsometry (or otherindependent method); (3) simulation of an experimentally ap-propriate monolayer spectrum (i.e., accounting for reflectiongeometry, etc.) using the above parameters; (4) assignment of eachabsorption feature to a single or combination of contributingvibrational modes. each possessing a matrix element conespondingto a transition dipnle moment u'ith a specific direction in molecularcoordinates: (-i ) asrgnment of the discrepancies in band intensitiesbetueen the iso t roprc s rmula t ion and the exper iment to averageonenta t rons o f each o i the e lements o i the t rans i t ion d ipo le mo-ment5 uslng a irameuork of classical electromagnetic theory; and(6) convert ing the dipole onentat ions in molecular coordinatesto a molecular geometr) ' on the surface via t ransi t ion dipole di-rect ions in molecular coordinates (Table I I ) , I t is , of course,cr i t ical that there exists a t ransferabi l i t i between the opt icalfunctions used (derived from a bulk state) and those relevant tothe phase state of the monolayer. Errors in the ell ipsometricallydetermined thickness are also of concern, although, for typicalerrors of less than several Angstroms, the influence on the de-termined orientations is generally small (values of thickness usedare given in the Experimental Section).

The procedure used for numerical comparison in the presentstudy differs in significant detail from that reported previously.The present study uti l izes complete spectral simulations; a de-scriptive overview of the method is given in the supplementarymater ia l ,

Simulations were performed for a wide range of surface ge-ometries of the adsorbed molecules. The range of structures tobe considered was narrowed somewhat by noting that the ex-perimental spectra (presented earlier) suggest the placement ofthe chains in a crystall ine-like environment. Indeed, it has beendemonstrated by several techniques that the monolayers on goldare ordered.l0'l l Therefore, only structures with alkyl chains inan all-trans conformations were considered initially. In the latterportions of the manuscript, we will present simulations in whichthis all-trans constraint has been relaxed to allow gauche con-formations at the terminal CH2 group. With regard to the presentfocus on all-trans chains, the copper system presents a significantchallenge to analysis. The best samples give spectra virtuallyidentical with those found on silver. Their analysis would thus

and Au Surfaces

lgls(CH2)nCH3

oo(g.ctL

oQtt

Cu/S(CH2LCH3 AuiS(CH.)nCHs

f o.oor

2800 2900 3000 2800 2900 3000Wavenumbers (cm'l)

Figure 4, IR spectra of monolayers derived from exposure of surfaces of copper, silver, and gold to n-alkanethiols (CH3(CHJ,TSH).

II

I

,i

2900

(40) Tables of the optical functions for all the alkanethiols reported in thisstudy (C,5-C2q and C22) are avai lable on request (D.L.A.) .

-€ n*/1 ̂ - \ / l

* -NV " \ -^ ̂ ./1

A'/V V \^a_r

il *

n hi lIV\J V \

^ J r -

^^ A/\l vw v \

-o-r:exp -er'exp i CH.(CHr)nS/Au- r ' r i scm-o r ' scm

-.-r:lcm -a-r'tcm

7 1 5 8 j l 3 , N o . / , 9 , l 9 9 l

Figure 5. Intensities of methyl stretching modes, r* and r,-, for n-al-kanethiols (CH3(CH2)"SH) adsorbed on gold and silver The experi-mental values of r"- contain a small contribution from the unresolvedintensities of the r5- modes. The upper panel displays experimental andcalculated intensity deviations (normalized to the band intensities foundfor a monolayer of hexadecanethiol adsorbed on gold) of the methylstretching modes, r+ (open) and r.- (filled), for n-alkanethiols adsorbedon gold. The calculated intensities are based on various structural modelsthat are discussed in the text: exp (experimental value), scm (single-chainmodel), tcm (two-chain model). The lower panel displays the experi-mental intensities of r+ and rr- for n-alkanethiols adsorbed on silver. Thelack of any change in the intensities of the symmetric and asymmetricmethyl stretching modes for odd and even values of n on silver sharplycontrasts that observed on gold (upper panel).

be that same as that which follows for silver and is therefore notrepeated. The "disordered" materials characterized by' the typeof spectra shown in the supplementary material cannot be treatedunder the constraints init ially imposed on this analysis, namely,that there does not exist a reference phase from which we canderive with certainty suitable optical functions; the spectra areneither crystalline nor liquid-like in character. The data suggestthat there exists in these materials some fraction of extended transsegments as well as considerable densities of gauche conformations.This might imply that lower coverages of adsorbate may be presenton these surfaces than is to be found on those bearing closestpacked chains. By assuming a coverage, the weighting of this fitas domains of l iquid and solid-l ike material could produce aconvergent simulation, but, given the i l l-defined nature of thesephases, there seems litt le justif ication for doing so.

Figure 6 shows a schematic diagram of an all-trans chain aswell as the coordinate system used to define the orientations ofthe chains. The chain ti l t angle, a, is defined as the cant of thelong axis of the trans-zig-zag polymethylene chain relative to thesurface normal direction (z). The chain twist, p, is the twist ofthe plane of the chain (*p, counterclockwise rotation) around thechain axis. This coordinate system, based on the typical Eulerconvention, is sufficient to describe the chain geometry of a un-iaxial system (recall that only the z component of the transitiondipole moments gives rise to absorption; all x and y orientationsare equivalent and thus the system is uniaxial), ln order todescribe both odd and even chains with this one diagram, the

Laibinis et al.

(5 t-:,

Figure 6. Schematic diagram of an all-trans chain in an n-alkanethiolsmonolayer on a surface. The coordinate system used to define the ori-entations of the chain, the cant angle a, and the chain twist 0 are shownalong with their relationship to the surface coordinates.

convention was adopted in which the metal-S-C geometry isconserved for both tvpes clf chains. The figure as drawn describesthe direction oi the C-CHr bond as a positive ti l t for a chain withan odd total number of carbon atoms (even number of CH2groups). The convention thus requires that a and B be specifiedfor an even chain in which the init ial C-CHr bond is t i l ted in theopposite (negative) direction.

The data above suggest that all the chain lengths examinedformed self-consistent structures. In the interests of brevity,therefore, fits to only a selected series of spectra are presented"We will consider first the molecular orientations of monolayerson silver. and then compare these to those on gold. For all thecases considered in the simulation models, excellent fits to theexperimental spectra could be found for the CH2 C-H stretchingmode adsorptions (d+ and d-). In no case, however, could asimultaneous convergence of the CH3 C-H stretching modes (r+,r.-. 16-) be achieved: the total errors could be minimized to valuesapproaching -20-30vc of the total methyl band intensities (seediscussion above). On the basis of the observed fitting errors, bestfit convergence is defined in terms of the d+ and d- modes, whichare used to determine the tilt (a) and twist (0) angles of the chains(see Figure 6), It is important to note that the symmetry of theCH2 groups about the chain axis precludes the use of the d+ andd- modes to determine the absolute sign of the tilt angle, a" Thislatter parameter, which establishes the structural features of thefi lm surface (i.e., the direction at which the terminal C-C bondprotrudes into the ambient), has been arrived at using observationsof the odd-even behavior of the CH3 mode intensities and wil lbe detailed below.

The errors associated with the best f its for a C1s thiolatemonolayer on silver are i l lustrated in Figure 7. The analysissuggests that these materials are, in fact, well modeled by theoptical constants of the bulk phase and exhibit a small but ap-preciable cant angle (a) of l 3o. The stiffness of the fit is clearlydemonstrated by the observation that variation of a and B by even*2o (not shown) greatly degrades the fit. The fits to the othersilver monolayer data sets were equally good with a range of theabsolute values of the cant angles (lal) varying from I lo (C,e)to l40 (Czo); the twist angles, B, were also similar ranging from42o to 44o (Cre). These variations are well within the accuracyof this method. Inspection of the data in Figure 7 also revealsthat the sign of the cant angle is strongly constrained by thesimulations. Even though the absolute intensity of the methyl

te

x

\ 2

oEdE lo r

oq ocEctlo

1 8'14

2.Ooo

: 1 . 56

EEot 1,0o?

1 91 4

rfl cH3(cH2)ns/As

. . Jl o

C a6 Hs7 SH /A9

^/1i t ' ,^r, l\

\!w

TIIIII

E

o = 1 3

B - - 4 2

: " \ / '

o.oor

= - 1 3= 4 2

Monolayers of n- Alkanethiols on Cu, Ag, and Au Surfaces

2800 2900 3000WAVENUMBERS ( cm-{ )

Figure 7. Experimental (lower) and calculated IR spectra of octade-canethiol adsorbed on silver. Calculated spectra having cant angles, c,of -l3o (upper) and +l3o (lower), and twist angles, S, of 42o arepresented. The differences in the calculated sp€ctra suggest that a is-l3o (as defined by Figure 6). Specific interpretations are discussed inthe text. The calculated spectra are based on a single chain model (seetext) .

modes suggests the importance of effects other than orientationin determining the band intensities (especially r6-). the fits failmost profoundly for positive values of a. This result. when viewedin the context of the data presented in Figure 5, suggests that thesurface projection of the methyl group does not vary in the sil-ver-supported monolayers as one changes the length of the chain.Stated differently, the fits, giuen the corwentioru adopted in Figure6, require negatiDe signs of a for odd numbers of methylenegroups and positiue ones for euen numbers. Discussion of theimport of this observation is deferred to later in the manuscript.

Whereas the spectral intensities of the monolayers formed onsilver could be fit by the use of a single-chain model, viz., oneaverage chain geometry representing the structure of all the chainsin the monolayer ensemble, the gold system requires the consid-eration of more complex models. In recent studies, it has beenfound that the spectra of these monolayers are strongly temper-ature dependent and reveal two important structural features.alFirst , the chains, even though wel l descr ibed as being'crystall ine-like", do indeed contain appreciable quantit ies ofgauche conformations; these are con@ntrated at the chain termini.Second, spcctra at low temperature exhibit factor group splittingswhich suggest that, at least at T S 220 K, the ordered structureadopted contains two chains per unit cell. These observations aresupported by recent molecular dynamics simulations on thismonolayer system.42 Thus, it is required from the outset thatthe models used incorporate two-chain geometries and allow thepopulation of terminal CH2 gauche conformations. Because ofthe demands placed on the accuracy of the spectral simulationsby such models to distinguish the subtle numerical differencesexpccted, added emphasis was placed on the measurement anduse of improved n, k data sets, data unavailable prior to this work(see above and supplementary material).

We start with a consideration of the simplest casc, a single-chainmodel. Using a best fit criteria based on the d+ and d- modcs,

(41) Nuzzo, R, G,; Korenic, E. M.: Dubois, L, H. / . Chem, p[vs. t990.93,767-713

(42) Hautman, J.; Klein, M.L. J. Am, Chem. Phys,19t9,9l, ,4994-SCol.

J . Am. Chem. Soc . , Vo l . I13 , No. 19 , I99 l 7159

2800 2900 3000W A V E N U M B E R S ( C M - { ) .

Figure 8. Experimental (lower) and calculated IR spectra of octade-canethiol adsorbed on gold. Calculated spectra having cant angles, a,of -26o (upper) and *260 (lower), and twist angles, p, of 52o arepresented. The two simulations provide equally good fits that do notallow absolute assignment of a. The calculated spectra are based on asingle-chain model (see text).

the single all-trans chain structure derived for n-alkanethiolatemonolayers on gold has an average cant angle, a, in a narrow rangeof 26-28o and a t r i is t . J. of -52-55o. Speci f ic s imulat ions areshown in Frgure 8 for an octadecanethiolate monolayer for bothpositive and negative values of a as well as the experimental datafor reasons of comparison. As before, the calculated data for thisparticular monolayer yields fits in a narrow range of values fora (shown here for a = *26o) and B (52o); changing these valuesby *,2o seriously degrades the quality of the fits for the d+ andd- modes.

On first glance, the data in Figure 8 might be construed asindicating that the proper value of the cant angle is a = -26o,

the fit to methyl 16- mde being somewhat closer for this sign ofa. The weighted errors are actually comparable in the two fits,when one takes proper account of the total intensity of the r+, r.-,and 16- modes. These specific simulations thus are insufficientin and of themselves to choose between the signs of a. The largerdata set (Cro-Czz thiols) does provide, however, a basis on whichto make such an assignment. The intensities of the methyl C-Hstretching modes fluctuate with varying chain length (Figure 5),revealing that the surface projection of the methyl groups is notconserved as one adds an additional methylene group to the chain;odd and even chains yield different methyl orientations althoughwithin these latter groups the structures appear to be similar. Thesimulations reveal that the qualitative character of these intensityfluctuations are best described within the confines of the single-chain model by assigning a positive sign to the chain cant angle,a (Figure 6), which is conserved for all chain lengths. We havenoted in an earlier publication that there exist significant per-turbations of optical functions of the monolayer for group locatedat the ambient interface relative to the bulk reference phase.l2'alThese effects are sufficiently large even for the fairly simple,nonpolar methyl group (intermolecular interactions by dispersionforces) so as to prevent definitive calculations of an average methylgroup orientation. This perturbation could involve in principleboth the magnitude and direction of the oscillating dipoles. Onthe assumption that the directions do not change, a simple cal-culation can be made from the experimental data as to the changesin intensity required to allow the exact quantitative representation

oE

E

(tto

otr(r

oro

Ca6H57SH /Au

a = - 2 6B = 5 2 1

7160 J. Am. Chem. Soc., Vol. II3, No. /,9, I99l

of a specific data set. The calculated modulations of CH3 C-Hstretching mode intensities shown in Figure 5 were derived on thebasis that the Au-S-CH2 bond angle is constant for all chainlengths. On this basis, the only difference between odd and evenchains can be the orientations of the C-CH3 bond with respectto the surface and, hence, the transition moment directions of ther+, rr-, and 16- modes. With these assumptions, the latter areexactly defined for both odd and even chains for a chosen tilt/twistconfiguration (Figure 6), and the ratio of the mode intensities canbe exactly calculated for an even chain versus an odd on the basisof the change in cos2 dr, (where 0r, is the angle of the transitionmoment with respect to z axis for a given mode of a particularchain length3T). The calculated moduli shown in Figure 5 werederived in this manner (the calculated changes in r"- and r* arereferenced to the intensities for the even chain). It is evident thatthe discrepancy is quite substantial. Most of this discrepancyapp€ars to be due to the assumption underllng the simple all-transsingle-chain model.

As has been already noted, the use of a twechain model is moreconsistent with current evidence that supports a low-temperaturestructure similar to a bimolecular monoclinic hydrocarbon phasewith the planes of the C-C backbone at -90o to one another.alFrom this starting point, two structural classes were developedfor spectral simulations: ( I ) the low-temperature structure withtwo contributing average chain structures, each of equal populationwith all-trans conformations; and (2) the high-temperaturestructure, identical with the above all-trans model except forallowing the population of both trans and gauche conformationsof the terminal CH2 groups of both chains in an independent.uncorrelated fashion.as

One distinct result obtained from the fitt ing procedure is thata family of structurcs all givc nearly- identical minimum error f itsto the CH2 modes for values of d boundcd by 0 S d < 82o '*' i theach value of d associated with a speci f ic pair of 0t ,02 values.For d > 82o, the fits worsen. Since the structure of a ortho-rhombic crystalline subcell would require d = 90o, the upper limitvalue of Q = 82o was selected as the most physically real value.{For this constraint, the all-trans, two-chain model yields chaintwists of 50 and -48o. The corresponding simulated spectrum,

(a3) To dcscribc the geometry of these models, the following conventionswcre adoptcd. For a givcn pair of chains, an anglc betwccn planes of thc chain,d, was selected and thc common axis between the planes allowed to rotate(equivalent to twisting thc chains in conccrt by thc same amount) and tilt awayfrom the surfacc normal. It follows that, for a pair of chains indexed as I and2, 6 = l1r - p2l and at = d2 = o. Sincc the planc of the chains can bc rotatedby l80o with no effect on the optical response of the CH, modes, one cancquivalently specify d as l80o - l0r - dzl. As a convention we choose thesmallest value of d, The stratcgy for thc bcst fit-simulation proccdures forboth models involved: (l) fixed a to bc that determined from the bcst fit ofthe single chain model (given above) and (2) variation of p1 and p2 indc-pcndently between *90o and -90o (one ncgative value of p is rcquired toproduce the proper methyl surface corrugation; sce below). In addition, forthe gauche-allowed model, 0r and p2 for thc bcst fit all-trans model weresclcctcd and fixcd while a largc variety ( - I d) of conformational populationswere scarched indcpcndcntly for cach chain in a collcction of 20 indcpcndentunit subcclls (2 chains pcr unit subccll). Thc monolaycr thus was reprcscntcdby 40 chains (20 with p; and 20 with pr), with each chain indepcndcntlypopulating one of three conformational geometries, trans, gauche (l), andgauche (2), with cach conformation exhibiting a different contribution tospcctral intcnsity.

(44) Thc numerical prccision in thesc calculations is such that a range of{ valucs bctween -75 to t5o (or equivalently l80o - (76 to 85o) = 104 to95o) will give a rcasonablc fit to the intensitics of thc CH2 modes. Thcrc isan intcresting and simplc gcometric rclationship that can bc dcrivcd rclatingthc twist angle of a single chain model, p,, to thc twist anglcs of the two chainmodel, p1 and 02, for thc condition that thc overall geometric factors (sccsupplementary material, footnote 7) for thc d+ and d- modcs, Gd' (a. d) andGo (o. d), cach bc idcntical, rcspcctivcly, in both models:

l9r- Bzl = c s lo(d, \ -2p.) (2)

In line with our earlier convention of choosing d, Q(p,) = 0o when Qo S 0,( 45o and o(0,) - l80o whcn 45o 3 P. < 90o, Thccondition requircd bythis incquality is that thc modcls givc idcntical simulatcd CH2 spcctra. Thus,for an orthorhombic unit subccll with d - 90o, it follows that p. - 45o andall two-chain models can bc rcplaccd by onc singlc-chain modcl. Anothcrrcsult is that for 9r= 52o, the averagc rcsult dcrivcd for a singlc-chain twiston gold in the tcxt, thc incquality rcquires 6 S 760, cl6c to thc rangc foundnumerical ly of -76-85o.

Laibinis et al.

CasH3TSH /Au

ALL - TRANSTWO CHAIN MODEL

a = 2 6F = 50 AND -48

GAUCHE . ALLOWEDTWO CHAIN MODEL

Q = 2 6I = 5O AND -48

Io oor

EXP.

2800 2900 3000

W A V E N U M B E R S ( c m - { )

Figure 9. Comparison of experimental (lower) and calculated spectra ofoctadecanethiol adsorbed on gold, The calculated spectra are based ona two'chain model The charns were assumed to be all-trans in the uppersimuiat ion and al lowed to adopt gauche conformat ions at the terminalmethl lene group rn the lower. Speci f ic d iscussions are presented in thel e x l .

AcFd)

Ac(3p)

6m5mtm3m2mlm0Blndlng Energy

(ev)Figure 10. XPS spectra of silver samples exposed to I mM solutions ofoctadecanethiol in isooctane for l, ll, and 37 h each. With continuedexposure, the sulfur signals become more intense with "growth" of peaksat lower binding energies (see inset); no other changes in the spcctrumwere observed. The S(2p) spectrum for the sample exposed to solutionfor 37 h could not be fit with the two spin-orbital-split components ofa single sulfur-containing species. This spectrum was, however, easilyfit by two pairs of spin-orbital-split components; one pair resembled thcspectrum given for l-h exposure suggesting that the thiolate remainspresent while an additional species (one at lower binding energy) hadbeen incorporatcd. We bclicvc the ncw sulfur-containing species tocorrespond to the formation of a thin film of Ag2S at thc silver/monolayer interface. The wettabilities of the samples were essentiallyindistinguishable and confirmed that the hydrocarbon films on them areoriented.

shown in Figure 8, while producing an excellent fit for the CH2modcs, clearly yields a poor fit to the intensity profiles of CH3

o(l

(r

oo

I

, , ,> o5 r E

!1,a

Erh1 1 ht h

1n 1Ca 101Blndlng En rgy

(d)

Monoiayers ai n-Alkanethiois on Cu, ,4g, und Au SurJaces

modes (indecd. oncs no better than the single-chain modeli. Usingthese f ixed va lues of a ,0r , and B2 which prov ide quant i ta t ivesimulation of the CH2 modes, the fitting proc€ss revealed a seconddistinct result, namely, that a large number of best fits to the CH3modes ex is t w i th near ly a random mix ture o f a l lowed gauchepopulat ions. ln the above, the worst f i t to the total intensity ofthe CH, modes is obtained for zero gauche states, viz., the all-transmodel above. The calculated spectrum shown in Figure 9 isderived from a model with a total conformer population of 45Vcgauche and 55Vo trans in the collection of 40 chains sampled. Ingeneral, even a small percent of gauche states can decrease thefi t t ing error associated with the CH3 modes by a t 'actor of -5.

I t should be noted that. of course. certain combinations of con-formational states give poor fits but none worse than the all-transmodel. Thus, randomization of terminal conformations can onlylead to improved f i ts. The f inal point to note is that no combi-nation of conformational states sampled could provide a less than- ljVo error between the spectral intensities of the calculated andobserved total CH3 modes.

I t must be emphasized, therefore, that precrse quanl i ta t i ( )n tobetter than *10% requires consideration of perturbi i t ions ir f theoptical constants of the CH3 group at the air lmonolayer interlacerelative to the CH3 group in a crystalline reference materral. Sucheffects probably exist and should be considered to be in thc rangeof - i A7o of ihe t<ltal values of k 1as suggested by, the fittrng errcrrsabove). The signif icant point remains, however. that the best f i tsare cons is tent wi th a popula t ion o f gauche conformat ions at thechain ends, as independent exper iments suggest exrs t .a5

Influence of Extended Immersion Times or Exposure to Air onthe Nature of Self-Assembled Monolayers on Silver and Copper.We have observed that, while the nature of the monolayers formedon gold are insensit ive to changes in the immersion t imes in theadsorbate solution over periods of several weeks, those formed onsilver and copper can differ in considerable detail. We have begunto examine these features in greater detai l and can report someinit ial f indings, Extended exposure of copper and si lver surfacesto so lu t ions conta in ing a lkaneth io ls resu l ts in f i lms that appeartoo th ick by e l l ipsometry ; the th icknesses in fer rec i are too largeto be exp la ined by adsorpt ion o f a monola ler (see above; . \ \ 'efound, by XPS, that the f i lms formed on s i lver . a f ter ex tendedexposure to the adsorbate solut ion, contained no ox)'gen and theamount of carbon was similar to that found on samples exposedfor shorter immersion t imes (Figure l0). The major dif ferencebetween the XPS spectra is the increased amount of sulfur presentwith continued expoaure to the alkanethiol solution.6 The amountof sulfur (relat ive to the amount of carbon) is too large to beexplained if sulfur is only present as alkanethiolate. We believeformal C-S bond cleavage has occurred to generate a thin filmof Ag2S at the silver/monolayer interface;a7 it may be this reactionthat is responsible in part for the anomalous el l ipsometricthicknesses mentioned earlier. Surprisingly, the wettabilities bywater and hexadecane of the hydrocarbon films remained similarto those obtained after more limited exposures and were consistent

(45) There is a further subtle correction in the simulated spectral intensitiesof the d+ and d- bands which should be made where gauche conformationsare introduced into the chain end. Because these modes are delocalized (seesupplementary material), the loss of one trans conformer in an all trans chainby conversion to a gauche state can diminish the overall band intensities byan amount greater than the fraction of the total CH2 group which the alteredgroup represents, For the case of an originally all-trans -(CH2),7- chainconverted to a I gauche k ink chain, thus we expect I t f I tS 17l18 = 0,94 ora more than 6Vo drop in intensity. A reasonable upper bound is probably- l0Vo (Snyder, R. G.; Maroncel l i , M.;Strauss, H. L. ; Hal lmark, V. M. " / .Phys. Chem, 1985, 90,5623-5631). Considcring that probably only half(-45Vo for the fit given in Figure 9) thc chains havc gauchc kinks, the cffectof the drop in intensity in the simulation is to increase the chain ti lt a anda change in the ti lt from 260 to -29o will accomplish this. Although sucha correction cannot be made rigorously because of the lack of actual gauchestatc populations and the exact intcnsity relationship, the sizc ofthe correctionis reasonable in terms of physically acceptable valucs of a.

(46) No differences were notcd in the character of the Ag(3d572) core levelupon extended exposure to the alkanethiol solution; however, the bindingcnergics of Ag(3d572) for Ag2S and bulk Ag are s imi lar (ABE S 0.1 cV). ' r

(4?) Cleavage of C-S bonds by silvcr of thiols, disulfides, and sulfidcs hasbeen reported.a

J . , -1 t r t t h t 'n t \ ' ' . , . I 1 e . 1 9 9 1 7 l 6 l

J o.oor

b

aI__T____r-]ffi

800 1000 1200 '1400 1600

Wavenumbers (cm'')Figure I l . I R specl ra of monolayers prepared on si lver by exposure tooctadecanethioi rn ethanol, after different intervals of subsequent expo-sure to the atmosphere (a, -l h; b, -2b). The high-frequency (upperpanel) and low-frequency (lower panel) regions of the IR spectra displaythe changes that develop with time. The high-frequency region showsthat the hydrocarbons chains have become oriented less perpendicularlybut remain in a crystalline-like environment. The low-frequency regionand the XPS spectra (Figure !2) show that some of the thiolate headgroups have been oxidizeci to sulfonates.

with the presence ci a ,Jensely packed. meth)'l surface. We believethat dur ing the formatron of . rn tn terphase of Ag2S, an organized,or rented a l l l r l th ro ia te assemblage cont inues to ex is t (see be low) .On copper. similar lncreases in sulfur content were observed,suggestive oi formation of copper sulfide; however, the surfacesroutinely became pit ted and did not al low characterization bywet t ing,

Upon extended expcsure of an alkanethiolate monolayer onsi lver or copper to the atmosphere. the sample changes; a rep-resentative example of the IR spectrum of such a material is shownin Figure I L This sample was obtained after a few days ofexposure o f an octadecaneth io la te monola ler on s i lver to a i r ,Three features of the data are of part icular signif icance. First,the peak positions of the CH2 C-H stretching modes are consistentwith the presence of a dense crystal l ine-l ike phase (d+, 2850 cm-l;d-,2917 cm-l). Second, the corresponding intensities suggest thatthe chains are more highly canteci than those described above.Third, the low-frequency port ion of the spectrum exhibits a veryintense band centered at - 1200 cm-r. Using the optical constantsof an n-alkyl disulfide as a basis set, we roughll estimate that thechain cant, a, for this new phase is - l8o with the twist E = 45o.We urge caution in interpreting this result. as the head group onthe chain is now presumably not thiolate (see Discussion below)and perturbations of n and & are expected to result. It does seemsafe to conclude, however, that the orientat ions of the methylgroups at the ambient surface differ in significant deuil from thooesupported by a thiolate head group,

The XPS data shown in Figure 12 for a monolayer on copp€rexposed to the atmosphere suggest that some of the sulfur atomsin this monolayer are oxidized as judged by the additional S(2pyz)core level at a binding energy of - 167.5 eV, An increase in theintensity of the O( ls) core level region is also observed. We believethe species present on the surface to be an alkyl sulfonic acid,presumably present as a sulfonate anion. The binding energy ofS(2pyz) is comparable in posit ion to that obtained by treatingan oxidized silver or copper surface with an alkyl sulfonic acid(see Table I). We have observed on si lver similar oxidation of

oocollL

oa

ll

7162 J. Am. Chem. Soc., Vol , I13, No. 19, I99l

173 168 163 158

Binding Energy (eV)Figure t2. Effect of exposure to atmosphere on XPS spectra of octa-decanethiolate monolayers on copper (bottom. (l h; middle, -2 wk).The upper spectrum is of a silver surface derivatized in an isooctanesolution of hexadecanesulfonic acid, presumably adsorbed as the sulfonateanion, and is provided for compar ison. Al l spectra (40 scans) * 'erere fe renced to C( l s ) = 284 .80 eV . The lower spec t rum exh ib i t s a S l l p tenve lope cons is ten t * i t h a th io la te a t tachment ; t he samp le e rposed toa tmosphere fo r I ueeks d i sp lays peaks assoc ia ted * r th the pa r t i a l a t -tachment o f t he mono laye r

" ' i a a su l fona te mo ie t l ( see Tab le I ) and

conf i rms assignments in Figure I l . XPS *as less sensi t ive to the presenceof oxid ized head groups that IR; longer exposures to atmosphere \ \ererequired to confirm the products of oxidation by XPS. Similar changesare observed in the S(2p) spectra of alkanethiolate monolayers on silveralthough they occur at a slower rate.

the adsorbed thiolates to sulfonates.2e The appearance of a strongpeak at - 1200 cm-r in the IR spectra of silver samples (FigureI I ) is entirely consistent with a sulfonate head group.

Discussion

The data reported here support a general understanding of thestructure of the alkanethiolate monolayers on copper, silver, andgold. The answer to the simplest question we can ask-*Are they'the same?"-is clearly "no". How they differ is easily describedin general terms,

Monolayers on Copper. All the evidence suggests that the filmsprepared on copper under the conditions we used (that is, byminimizing, but not completely excluding, exposure to laboratoryatmosphere by manipulating the unfunctionalized metal undera flow of argon) can vary widely in character. The significantvariations of spectral band intensities and line shapes all suggestthat some factor related to the structure of the monolayer com-plicates this system. The clear inference from the XPS data isthat the variations in the surface of the starting substrate, dueto adventit ious oxidation and other reactions with atmosphericconstituents and, perhaps, to differences in the morphology of thesurface, are l ikely underlying factors.

The copper surface that supports these monolayers is clearlynot metallic. The O(ls) core level data establish that the coppercontains oxygen. No Cu(ll)(2r1,D satell i te structure could bedetected by XPS; thus, the most likely phase present is a hydroussurface oxide related to Cu2O. This oxide surface is also knownto be highly active in the sorption of environmental contaminantsas well as in the strong adsorption of CO2 as carbonate. Themicroscopic topography of this surface is unclear. It is thuspossible that any one or all of these factors could conspire topreclude the adsorption of thiols to the very high coverages or inthe geometries necessary to form a structure comprised ofclose-packed and, perhap, ordered alkylchains. It is not surprisingthat copper samples prepared using apparently indistinguishable

Laibinis et al.

protocols yielded monolayers exhibiting various degrees of con-formational disorder by IR and complementary differences inhysteresis of contact angles of water (see supplementary materials).

On clean elemental surface,s of Cu( I I I ) under UHV conditions,s imple th io ls and d isu l f ides (CH3SH and CH3SSCH3) adsorbreadily.a8 The surface species generated from either adsorbateis a surface thiolate. CH3SCul.Cuo,. The structure of this organicsurface phase on Cu(l I l) has not yet been determined by LEED,but the evidence obtained from angle-resolved photoemissionmeasurements supports a highly defined structural model con-taining a S-C bond projecting along the direction of the surfacenormal. Such a surface phase is qualitatively consistent with thecharacter of the best monolayers we have prepared on copperalthough the values of a found suggest a somewhat differenta l ignment o f the a l l - t rans cha in (a = l2o) . In most ins tances,however, the data obtained in this study do not support a structureresembling the well-defined structure obtained in UHV. Wetherefore suggest that the oxide surfaces present in experimentsinvolving adsorption from solution must seriously complicate theformation of dense organic phases. Under certain conditions thisoxidation might be reversible and/or the Cu atoms removable asions (Cu(*l)) to al low the formation of the dense phases seenon occasion. As noted, though, the experimental variable(s)involved remains undefined. Further oxidation of the surface afterformation of the monolayer, and, possibly, oxidation of the thiolatesas well . may also contr ibute to the disorder in the monolayer.ae

The XPS data clearly suggest that the major sulfur speciespresent i - . a th io la tc (RSCu) : the S(2p) core leve ls (Table I ) aresh i f ted br , ) rer I eV to iower b ind ing energy f rom the pos i t ione rpcc t cd l \ ) r l t h l o i t u i t h t hc - S -H bond i n tac t ) . The same so r ttrf :p€clc: it)f ms ,]n elemental surfaces in UHV.47 The microscopicmorpho log r o f t he :u r f aces may be ve r l ' d i f f e ren t , and t he i n -f luence on s t ructure o f sur face s i tes occupied by ox ide ions andbi small cluanti tres of alkyl sulf inic or sulfonic acids is presentlyunclear. The XPS data also strongly suggest that the bondingat the surface can be heterogeneous (a broad S(2p) envelope issometimes seen). Whether this heterogeneity is attributable todifferent sulfur valence states or metal bonding sites is unclear.

Monolayers on Silver. In contrast to the copper-supportedmonolayer, the monolayers formed on silver are remarkablywell-defined, and the assembly of monolayers on this metal betterbehaved than on copper. We would expect that silver, which alsoforms an oxide in air, might show complexit ies similar to thosefound for adsorption on copper. The core-level data clearly show,however. that nearlv al l of this oxide must have been removedon the adsorption of the alkanethiols and that oxidation subsequentto format ion of the monolayer must be s low. We do not knowat present where the residual oxygen detected by XPS remains,but suggest that grain boundaries are at least one likely candidatesite. The data also show that this surface phase, when exposedto the atmosphere, gradually converts to what we believe is amaterial attached as a sulfonate; the O(ls) core level intensityseen at short immersions may, in fact, correspond to a partialsurface coverage of this latter ligand. The signal-to-noise presentin the XPS measurement makes the S(2p) core levels of this lattergroup hard to detect. The IR data require a structural model forfresh (not aged) monolayers on silver in which the chains are verydensely packed. The line widths, the peak positions, and the shapesof the d- and d+ modes resemble those expected for a crystallinehydrocarbon solid to a remarkable extent. The structure wepropose is thus one based on an all-trans chain with very fewgauche conformations. The quality of the methyl band fits sug-gests that gauche conformations are probably less important inthe phase diagram of these phases at 300 K than they are in thecorresponding structures on gold (see below). The simulat ionsin Figure 7 also define a structure in which the main chain axis

(4E) Bao, S,; McConvil le, C. F.; Woodruff, D.P. Surf. Sci.19t7, 187,133-143, Prince, N. P.; Seymour, D. L.; Woodrufl D. P.; Jones, R.G.;Walter, W. Suy' Scd, 1989, 215, 566-576.

(49) The bound thiolate and underlying copper surface each oxidize withextended exposure of the monolayer to atmosphere: Laibinis, P. E.; Whitc-sides, G. M. Unpublished results.

1 i so'-xo 4tr=o9 3.=o

52s

Monolayers of n- Alkanethiols on Cu, Ag, and Au Surfaces

Figure 13. Space-fi l l ing model of an octadecanethiolate monolayer onsilver as derived from infrared data interpreted in terms of a single-chainmodel.

is only very slightly canted from the surface normal direction. Agraphical depiction of this structure is shown in Figure l3 for anoctadecanethiolate monolayer. These results implicate a significantreduction of the interchain spacing in these phases in comparisonto the analogous ones that have been extensively characterizedon Au( l I l ) in th is and other papers. On this lat ter surface. wenow know with great certainty that canted phases (a = 28o withthe chain rotation p = 52o, see below) are formed whose characteris well suited to the very strong tendency of this system to formordered (J3 x V3)R30" sulfur overlayers. We believe that thecanted surface phases seen on gold reflects the spacing of sulfurin this overlayer structure. The differing character of similarmonolayers on silver, despite the similar metal-metal spacing forsilver and gold, suggests that the same ordered overlayer is notformed on silver.5o

J. Am. Chem. Soc., Vol. I I3, No. / ,9, l99l 7163

Comparisons between Monolayers on Gold, Silver, and Copper.The differences in geometrv (a and p) of the hydrocarbon portionof the assemblies on the coinage metal surfaces can be consideredto arise from the balance of free energies from three sources: (l)the adsorbate/subst ra te in ter face, (2) the cha in-chain pack ingof the in ter ior o f the assembly , and (3) the methy l in ter face (orsolvent/methyl interf ace). The energetics of the first contributionabove have been examined in the case of gold3o (the energeticson si lver and copper are no doubt similar) and are l ikely to belarger in magnitude than is possible from the energetics of hy-drocarbon chain interactionssl or hydrocarbon surface energies.Therefore, it is likely that head group/substrate energetics drivethe major structural features of the entire assembly. After thisconsideration, one could ask the question of what structure of thehydrocarbon chain (tilt angle and packing) and what arrangementof surface methyl groups (orientation relative to the surface andto other methyl group) represent minimum free energy structuresgiven no constraint of surface/adsorbate bonding, viz., no prefenedlatt ice spacing or valence angle. This question is impossible toanswer experimental ly at present, but, as judged by the infrareddata. the two d i f ferent c lasses of r le l l -def ined monolayers weobsen 'e (go ld : r i i rer and coppe r ) both exh ib i t dense, crys ta l l inepack ing. and rn thc case o i go ld . some composi t iona l d isorder ingat the alr (or solvent) interface. The energy dif ferences betweenthese tuo classes of f i lms. although indeterminate experimental ly,may be as small as dif ferences between orthorhombic andmonoclinic phases of hydrocarbons.aa Given this negligible dif-ference and also given such a large discrepancy between the totalfree energies of the adsorbate/substrate interface and of theinterior and the surface of the film for a particular system, itappears that the most likely factor determining the differencesbetween two films formed on different substrates must be primarilythe substrates themselves. On this basis, we consider the headgroup/substrate structure in detai l f i rst.

As has been noted. the XPS data indicate that the bonding tothe surface is in the form of surface thiolates for the three metals.The l i te ra ture contarns consrderab le precedent for the bondingof sulfur-containing species on si lver in this manner. On Ag(l I l )( the preferred crystal lographic texture of the polycrystal l inesamples used in this study; see Experimental Section), elementalsulfur and H25 have been found by LEED to form a number ofstable, ordered surface phases.s2 The preferred habit is a ({7x \/1)Rl0.9o overlayer. This dense and structural ly complexphase is believed to correspond to an ordered, lattice-matchedinterface between Ag(l I l) and (l ll)-oriented 7-Ag2S. The silverin the f irst layer is bel ieved to be ionic. Dimethyl disulf ide dis-sociatively chemisorb,s on Ag( I I I ), and the surface thiolate formedalso adopts the (t/1 x V7)Rl0,9o structure.s3 The S-S nearestneighbor distance in such a structure is -4.41 A. I t would thusseem that the bonding of sulfur in this sort of ionic surface phase(one that signif icantly reconstructs the ( l I I ) latt ice plane) isconsistent with the character of the phases inferred by infraredspectroscopy. Molecular dynamics calcu I a t i ons s s u ggest, however,that this lattice may be too dense (16.87 AulnS) to accommodatethe al l- trans chain sterical ly. The other phases seen in the H2S

adsorption studies,s2 most notably the r '39R I 6. I o x /39R16.1 o

overlayer (a structure corresponding to an ordered (100) layerof 7-Ag2S), could also support the adsorption of the sulfur ligandsat dens i t ies -8Vo h igher ( -20,3 A2, 'RS; than those in the (V3x V3)R30o overlayers found on gold. The small angle of cantderived from IR data. we bel ieve. ref lects the short interchainspacings imposed by the si lve175u1;ur latt ice. LEED or other

(51) The group const i tuent heat of subl imat ion of normal hydrocarbonsis 1.8 kcal /mol CH2 at 0 K: Salem, L. J. Chem. Phys.1962, 37,21CG2113.For very long hydrocarbon tails (n I 20), thc enthalpies due to chain packingand the metal-head group interaction may well bc of comparable magnitudcs.

(52) Schwaha, K. : Spenccr, N. D. ; Lambert , R. M. Sr l .Scd. 1979, 8/ ,273-284. Rovida, G.: Pratesi , F. Surf . .Sci . l9t l , 104,609424.

(53) Harr is , A. L. : Rothbcrg, L, ; Dhar, L. ; Levinos, N. J. ; Dubois, L. H.Phys . Reu. Ittt . l9XJ. 64, 2086-2089. Harris, A. L.; Rothbcrg, L.; Dhar, L.;Levinos, N. J.; Dubois, L. H. J. Chem. Phys. 1991,94,2438-2448.

(54) Bareman. J. P. ; Kle in, M. L. J. Phys. Chem.l9{n,94,5202-5205.

(50) Portcr et al. have also characterized the structure and properties ofa-alkancthiols adsorbed on evaporated silver films by wetting, ellipsometry,and IR (Walczak, M. M.;Chung, C. ;Stole, S. M.;Widr ig, C.A. ;Porter , M.D. J. Am. Chem.,Soc. t99l, I I 3,237F2378). A comparison with the resultsreported here for silver indicates general agre€ments between the sets of data,but shows differcnccs for monolayers derived from shorter n-alkanethiols. Forelqr_nRlcr_1vc.do not observe an odd-vcn effect in the wettability of Ag/S-(CH2)"CH3 by hexadecane (n = 4-8); Porter et al. do. They conclude thatthe hydrocarbon chain is canted at -*l3o for all ualues of n;we contendthat the sign of the cant angle, a (Figure 6), alternates bctween positive andnegativc valucs for even and odd values of n, respectively. We bclieve theiranalysis to bc in error. In both works, no variation in the intensities of methylstretching modes was obscrved for n 2 5. For shorter chain length (n = 4-7),Porter et al. obscrve as much as a 4Mo increasc in thc intensity ol these modes.Our spectral simulations (Figure 7) suggest this variation ii too small to bcdue to odd-cvcn variations in chain lcngth; we cstimate that an -l00%oincrease in intensity is required. They conclude an odd-cven effect is re-sponsible for this variation but do not offer satisfactory explanation for itsdisappcarancc at n > 8, Wc rationalizc this constant intcnsity by the existcnccof a single mcthyl orientation on silver as n is varied due to-an'alternation inthc-sign of thc cant anglc, a (Figure l4). Portcr ct al. also usc correspondingodd-even variations in wettability for n = 4-8 (that we do not obsirvc) aicomplementary evidence. We suspcct most (if not all) of the differencesbctween our work and that of Porter et al. to bc due to adventitious (andpcrhaps contaminant promotcd) oxidation of their evaporated fi lms of silverprior to adsorption of the alkanethiol, and to the sensi[ivity of this system tosurfacc oxidation when onc uscs shorter (n s 6) n-alkancthiols. we notc thatboth papers have had difficulty preparing high-quality monolayers fromshorter chain alkanethiols. we cannot, howcvei, eicludi some contributionto differcnces in the procedures used to prepare the sitver films, and to re.sulting differences in grain size, preferred orientation, and surface topology,

7164 J. Am. Chem. Soc., Vol . l l3, No. i ,9, 1991

diffraction studies will be necessary to define the structure of thisunderlying lattice unambiguously.

Again, as is the case with copper, the mechanism of the ad-sorption of alkanethiol on silver is unclear. The oxidation of oneor two monolayers of silver at the solid-air interface should befacile under the conditions used in the preparation of these samples.It is also clear from simple thermodynamic arguments both thatan Ag(0) surface should be able to dissociate the S-H bond andthat AgO, species should be reducible by thiols. Given thatmetathetical replacements of oxygen atoms on some silver surfacesare also facile, it is easy to envision a number of processes leadingto the materials seen experimentally. This latter chemistry alsosuggests another reason for the higher adsorbate densities whichform on silver as compared to gold. The init ial oxidation of thesubstrate by Oz yields a high coverage of oxygen atoms on thesurface.lT Their substitution by sulfur would thus directly yielda surface phase of dense chains.55 Even so. the precedent es-tablished by the UHV studies of dimethyl disulfide suggests thatthe formation of thiolates in high surface densities is intrinsic tothe Ag( l I l ) lat t ice plane.

As for both copper and silver, the bonding to gold also appearsto be as a surface thiolate as judged by the core level shifts seenin the XPS data. The placement of the sulfur atoms in a (r/3x /3)R30o overlayer, as mentioned above, is now well estab-lished.l0'll The picture we envision is one involving a more covalentstructure than that on silver. On gold, there are no large dis-placements of the metal atoms from their normal ( I I I ) latt icesites. The constant chain cant angle seen in the gold systempresumably reflects the dominant energetics of the metal-sulfurbond. The order ing seen ref lects the interpla;- of the dominantenerget ic contr ibut ions of the metal-sul fur bonding and thesecond-order energetics of the packing of the alkyl chains. In thecase of gold, the covalent characler of the substrate bonding resultsin the adoption of a fixed Au-S-CHt valence angle, providingan interesting contrast to the behaviors seen on copper and silver.

Havi ng discussed the dominant adsorbate/ substrate interaction,we now turn to the issue of f i lm structure. An intriguing pointarising from the IR analysis is that the methyl mode spectra ofthe films formed on silver distinctly do not reflect changes in theabsolute sign of the chain cant angle, a (Figure 6), with changingcarbon chain length. If the absolute direction of the chain ti l twere conserved with odd-even variation of length, the characterof the spectra in the region of the methyl modes would varlconsiderably and be quite observable (see Figure 5). The presenceof a disordered methyl surface, such as exists on gold, woulddiminish this effect somewhat, but the quality of the simulationsin the region of the r+ and rs- modes is sufficiently good (Figure7) to indicate that gauche conformers are less important on silverthan on gold. Using the conventions of orientation given in Figure6, the chain structures on silver (and likewise on copper) aredescribed in terms of ones in which the chain is required to cantin opposite directions and the Ag-S-CH2 ualence angle to al-teruute between two different sigtu of a for odd and euen numbersof methylene groups (Figure l4). The methyl surface projectionselected by each is the same dense phase, one which we cancharacterize as being more like a surface ethyl group (Figure l3)maximizing nearest neighbor interactions at the chain termini.Although the energetics associated with this methyl surfacestructure should be small, they seem to be sufficient to outweighany preferences due to the sulfur, We believe this picture to beconsistent with the hypothesis that the bonding of the sulfur atthis surface is largely ionic.53 With the above considerations in

(55) Scc, for cxamplc: Wachs, I. E.; Madix, R. J, Surf. Sci. 191t,76,531-558. Sexton, B. A.;Madix, R. J. Surl Sci. t9t l , i ,05,177-195. Stuve,E, M.; Madix, R. J.; Sexton, B. A. Chem. Phys. ktt.19t2,89,48-53, Stuvc,E. M.; Madix, R. J.: Scxton, B. A, Surf. Sci. 19t2, I 19,279-290. Canning,N. D. S.; Madix, R. J, /, Phys. Chem.1984, 88, 2437-2446. Fcltcr, T. E.;Wcinbcrg, W. H.; Lastushkina, G. Y.; Zhdan, P. A.; Borcskov, G, K.; Hrbck,J, Appl. Surf, Sci. t983, /6. 351-364. Au, C. T.; Singh-Bopari, S.; Robcrts,M. W,; Joyner, R, W. "/. Chem. Soc., Faraday 19t3,79, 1779-1791, Capote,A. J,; Madix, R, J, Sry' Sci. 19t9, 214,276-288; J. Am. Chem. Soc. 19t9,I I I ,357G3577. Brainard, R. L.; Madix, R. J, /, Am. Chem. Soc. 1989, / / /,3826-3835 and rcfercnccs citcd thcrein.

n = even

o - -27'

(not formod lorn - odd or even)

Figure 14. Illustration of the canted structures formed upon adsorptionof n-alkanethiols. CH3(CH2),'SH, on copper, silver, and gold. Thestructures that form on copper and si lver do not exhibit any change inrhe rntensrtres of methyl modes with incremental increase in the numberof methylene groups (Figures 4 and 5). This observation is consistentonly *rth the formation of a structure rn which the cant angle, a, ispositive for an even number of methylene groups and negative for an oddnumber oi methylene groups; the geometry of the metal-thiolate inter-action on these metals is not fixed as n is varied (upper panel). Thestructures formed on gold exhibit an odd-even modulation in the inten'sities of the methyl modes with increasing chain length (Figures 4 and5), This modulation reflects the formation of a structure in which var-iations in the chain length of the adsorbate do not perturb the geometryof the gold-thiolate interaction ( lower panel).

mind. we conc lude that assembly o f a g iven f i lm proceeds so asto satisf l the adsorbatmubstrate energetics f irst and then thoseassociated r i i th the methyl (ambient) surface ( in the form of theambient f i lm sur face tens ion) .

The structures formed on gold are understood quite consistentlywith the above principle. The placement of the sulfur atoms ina (J3 x v/3)R30o overlayer with a f ixed Au-S-CH, valenceangle forces the chains into the same absolute orientation re-gardless of chain length. As a consequence, the orientation ofthe methyl group modulates with carbon number (Figure 5).There are, however, further subtleties to these structures. Wealso know, both from experiment and theory, that the lowest energystructure for this system does not involve a simple hexagonal latticcof the chains but, rather, is one reminiscent of the bimolecularmonoclinic phases of the normal hydrocarbons.s6 We also knowthe symmetry of the methylene subcell cannot be simply ortho-rhombic as evidenced by the optical anisotropies in the infrareddata (an orthorhombic subcell , at any value of a, would yield anaverage 0 = 45ou and be accompanied by d+ and d- intensityratios approximately a factor of 2 different from those seen ex-perimental ly). To this point, both experiment and theory havefailed to establish how important this two-chain unit cell is to thestructure of the monolayer at temperatures greater than 220 K.The simulations presented in Figure 9 suggest that the structurepresent at 300 K may contain still significant signatures of thistwo-chain subcell . Our results also show that the IR data areconsistent with, but do not independently establish, significant(perhaps as high as 45Vo) conformational disordering of the end

"f" )'r'\

) , = , ,,J,;,X;, \ )\t

HH

' H H45"\ (

o = - ' 2 " ( ( r : ; : ; ; ,

t /) )

(56) Shearer, H, M. M.; Vand, U, Acta Crystollogr.l95S, 9, 379-384.

Monolayers of n- Alkanethiols on Cu, Ag, and Au Surfaces . l Am. Chem. Soc. , Vo l . I l3 . No. 19, lgg l 7165

stricted and therefore homogeneous methyl surface. It seems mostinteresting. given rhcse very dif ferent surface structures, that ad i f ference rn uer tab i l i ty is not observed.

It fol lows br rnference from the latt ice arguments above thatthe structures f()rmcd on gold can accommodate structural per-turbations that arc preciuded on silver. The reconstruction of themethy l sur facc. u hcther dr rven thermal ly or in response to acontacting external phase. r. aifected mechanistically both by thepopula t ion o f gauchc conformers and rhe un lock ing of the cha int i l t d i rec t ion and magni rude t :

The canted s t ructures on go ld . in cont rast to those formed oncopper and s i lver . might presuntab l r accommodate some par t ia lin terca la t ion o f hexadecane b1 uncant ing cha in segments . Theamount o f vo id space generated b1 uncanr ing rhe hrdrocarbonst ructure would be much greater on go ld than s i lver : fur ther .negl ig ib le space is probably ava i lab le on s ih 'er s ince the chainsare, prior to contact with a liquid, oriented near the surface normal(a = l2o) . In terca la t ion o f hexadecane in to a monolaver wouldresult in a lowering of the advancing and receding contact anglesof hexadecane and occur. based on structure. to the greatest extentpossible on rhe materials formed on gold. The contact angles andhrs teres is o f heradecane on the monolayers formed on s i lver andgo ld a re . r n i ac r . i nd i s r i ngu i shab le : ue t hus conc lude t ha t i n te r -ca la t ic rn t r f heradecane rnro monola lers on go ld does not occur .Bain ct al found rhar alkanethiolate monolayers on gold preparedin hexadecanc \ {ere not d i f ferent (by e l l ipsometry , XPS, andwett ing) from monolayers prepared from other solvents. Thisobservation suggests that, even in the presence of a large excessof hexadecane, intercalation of solvent had not occurred,20

On gold, the orientation of the methyl groups at the mono-layer-air interface is highly dependent on the number (odd versuseven) of methylene groups in the adsorbate: monolayers on silverdo not exhibit any orientational variation with chain length (Figure5) . The s t ructura l d i f f "erences betueen these two monolayers \s tems a i lou 'contpr ; is6n 6f an "odd-even" sens i t ive s t ructu fer . n go ld u i i h t he ' t , dd - e ' , ' cn ' neu t ra l s t r uc tu re on s i l ve r as t heyre la te to a \ur f rcc-sens i r i re proper t ) , wet t ing. The wet tab i l i t iescf r rs ls i and hcradecane both monoton ica l ly increase wi th in-creas ing chain length. no per iod ic t rend is observed on go ld (ors i lver ) . I t is wor thwhi le to ask, in v iew of the magni tude of thedifferences in methyl orientat ion on gold, why there appears tobe no inf luence on wett ing. I t is conceivable that the number ofgauche conformers on gold, part icularly in the region in contactwith solvent, is suff icient to lessen the magnitude of the corru-gation. There is, of course, no requirement that the structuralcharacterist ics of the monolayer when in contact with air (orvacuum) are preserved when the monolayer is placed in contactwith a liquid. It is also entirely possible that hexadecane and waterare sensit ive to more than the -0.5 A that would be required toobserve the small dif ferences in structure that arise from con-formational dif ferences in the methyl groups.

In general, measurements of wett ing on these interfaces wereextremely useful in defining monolayers that were disordered orrough. They did not, however, al low discrimination between thetwo types of ordered structures observed here, even when thestructures substantially affect (on the molecular level) the structureof the monolayer-vapor interface. whatever dif ferences existbetween gold and si lver in such structural detai ls f ind l i t t le ex-pression in wett ing. Other probes of the structure of thin f i lms(for example, helium diffractionr') ma) prove to be more sensitiveto these sorts of perturbations.

Conclusions

n-Alkanethiols spontaneously' assemble onto surfaces of copper,silver, and gold and form densely packed, oriented monolayer films.The films formed on silver and copper differ in structural detailfrom those formed on gold: the hydrocarbon chain is orientedmore perpendicularly with respect to the surface; the films (whencareful ly prepared) exhibit a lower populat ion of gauche con-formations at room temperature; the orientat ion of the carbon-sulfur bond relative to the surface plane varies with chain length.The formation of reproducible films on copper and silver is com-

Figure 15, I l lustrat ion of the structure of a n:onora 'ver o i an ( icrade-cane th io la te mono laye r on go ld i n fe r red f rom a n o -cha in ; i n ru la t i onbased on a l l - t rans cha ins , The inse t shows the re ra t i onsh ip o f an o r rho -rhomb ic cha in sub la t t i ce (uncan ted ) re la t i ve ro the he ragona l su l fu rsublat t ice and the di rect ion of the cant of the hydrocarbon chain ior th isarrangement as inferred f rom IR spectroscop) ' .

chain segments at 300 K. These'quant i tat ive 'values cannot betaken too l i teral ly . however, g iven the s igni f icant perturbat ionsof the methyl group t ransi t ion moments noted in the data. Aschemat ic depict ion of an ideal ized two-chain structure on goldis shown in Figure 15. The reader should note that this structurederives from only one fit to the data, howbeit one we believe tobe plausible on the basis of steric arguments. Diffraction studieswill be required to settle the importance of the two-chain unit ceilat room temperature.

Structural Effects on wetting. The strucrures on the threemetals differ in significant ways that may affect their respectivewettabi l i t ies. A s igni f icant issue that now exists for orsanizedfilms is what are the detaited relationships between rnJlecularfeatures at surfaces, such as orientation and corrugation, andwetting. ,Wetting is influenced by a number of factors, amongthem molecular-scale and macroscopic roughness.5T The mon-olayers we prepared on copper were of variable quality, in partdue to morphological changes that occurred duiing oxidationandf or subsequent adsorption of the alkanethiol; monolayers onsilver, in contrast, were more reproducible. Structurafly, ourhighest quality monolayers prepared on copper were similar tomonolayers prepared on silver. Given this similarity. we restrictour analysis to a contrasting of the structure-property relationshipin wetting of monolayers formed on silver and gold.

The wettabilities of monolayers formed on silver and gold exhibita common increase in hydro- and oleophobicity with increasingchain length (Figure 2). Similar progressions have been observedwith alkanoic acids adsorbed on alumina.3T The lower hydro-phobicit ies of monolayers prepared from shorter alkanethiols (n( 9) have been rationalized previously in similar systems byassuming disordering of the shorter chain adsorbates and inter-action of the probe liquid with the underlying metal sub,strate.8,e,37Thelimiting contact angles for monolayers derived from long (n> l5) alkanethiols are, within experimental error, the same onthe three metals examined and are in accord with varues reportedfor oriented alkyl monolayers on other substrates.E.zl-zs 16i 61r,on silver, which we believe contain a smailer number of gaucheconformers than those on gold, might be expected to prisent alower energy interface expressing a more conformationally re-

_ .(57) Wenzel, R. N,.lnd, Eng. Chem. lggfi, 28,9Eg-994. Dettre, R. H.;Johnson, R, E. Adu. Chem. Set. 19d4, No.43,136-144. Joanny, J. F.; deGcnnes, P. G. /. Chem. Phys. 1984, 81, 552-562. De Gcnncr, p. C. Rrr.Mod. Phys, 19t5, 57, E27-863.

7166 J. Am. Chem. Vol. I I 3, I A , I 9 9 I

plicated by the sensit ivi ty of these systems to exposure of the

unfunctionllized or functionalized sub,strate to air, and to extended

exposure to the solution containing the thiol. Films of high quality

ari , ho*euer, readi ly obtained with careful attention to detai l .

Extended exposure to alkanethiol solut ion results in reaction of

the alkanethiol with the surfaces of silver and copper to form an

interphase of metal sulf ide on which an organized, oriented

monolayer is supported. Although the systems of alkanethiols

assembled on copper and si lver are more dif f icult to work with

than the corresponding systems on gold. this set of systems offers

the opportunity to explore the effect of structural differences on

a number of propert ies including wett ing, intercalat ion, and ca-

pacitance.

Experimental SectionMaterials. Materials were obtained from Aldrich and '.tsed as received

unless specif ied below. Nonyl, undec.v-l (Pfr i tz & Bauer). tet13denl(Pfaltz & Bauer). and octadec-vl thiols were dist i l led under reducecipressure prior to use. Heptadecyl. nonadecl i , and docosl l thiols uc'reavailable from previous studies.E'i3 Pentadecanethiol was supplied by ourco l league Kev in L . Pr ime (Harvard) . Hexadecanesul fon ic ac id wasobtained by acidif icat ion of sodium hexadecanesulfonate with sulfuricacid and purified by recrystallization from hexanes. Octadecanethiol-duwas prepared from the corresponding perdeuterated bromide (Cambridge

Isotopes, 98Vo D) by nucleophilic displacement by thioacetate and sub-sequint methanolysis with K2CO3/MeOH; the product exhibited TLCbehavior identical with that of octadecanethiol and formed monolayerswith indist inguishable propert ies (wett ing, XPS. and el l ipsometricthickness (for monolayers on gold)) from those fcr:med fr6m octadecan-ethiol. Solvents were obtained from Mallinckrodt and deoxvgenated bvbubb!ing N2 for 30 min unless otherwise specif ied belor ' gr in the tett .no fur ther e f for ts were used to exc lude a i r . Isooctane t \ ld r ich) uaspercolated twice through neutral alumina; ethanol (abs) *as obtainedfrom Quantum Chemical Corp. The silver and copp€r (Aldrich) and gold(Materials Research Corp.) used to prepare eyaporated sampies were of>99.99Vo purity. Si l icon (100) waiers (100 mm) r, lere obtained iromSit icon Sense and washed with absolute ethanol. Prepurif ied Ar(>99.998Vo, <5 ppm 02) was obtained from Med-tech.

Sample nepandon. Filtt of the various metals ( 1200-2000 A) wereprepared by evaporation subsequent to deposition of 200 A of chromiumor titanium (the latter being preferred) onto silicon wafers: evaporationof silver and copper were onto precut I X 3 cm2 slides of silicon as wellas onto silicon wafers; Au was evaporated onto silicon wafers and, in somecases, subsequently cut into slides. The evaporations were conducted ineither a cryogenically pumped electron beam chamber (base pressure -8

x l0-E Torr). a cryogenical ly oumped resist ive boat chamber (basepressure *2 x l0-8 Torr), or a thermal evaporator evacuated with ad i f fus ion pump (base pressure o f ( l X l0{ Torr ) ; on l i ' monolarersprepared on copper were sensitive to the evaporator used. For e!'apora-t ions of copper. the chamber was backfi l led with prepurif ieC argon im-mediately fol lowing evaporation (prolyethylene bags * 'ere place aroundthe openings of the evaporator to minimize dif fusion of air intc thechamber), and the substrates were transferred under a flow of argon tosolut ions in disposable scint i l lat ion vials. Fi lms of si lver were preparecieither as described for copper or by backfitl with N2 and transfer of thesubstrates in air to solutions in Teflon containers. The backfill andtransfer procedures typically took at the most 2 and 5 min. respectively.

Slides were removed from solutions after the stated duration of ex'posure. washed with absolute ethanol, blown dry with N2, and charac-terized. Samples used for IR were prepared on 2-in. Si wafers andfunctionalized in Petri dishes.

The alkyl sulfonate samples were prepared by transferr ing ireshlyevaporated samples of copper and si lver, in air, to solut ions of hexade'canesulfonic acid (- I mM) in isooctane. After - l2 h of immersion,the samples were washed with hexanes, blown dry with N2, and char'acterized.

Morphology of the Substrate. The crystallographic textures of thehighly ref lect ive polycrystal l ine substrates supportcd on Si(100) used inthis study were determined by X-ray dif fract ion. Two independentanalyses were routinely made, The first (a socalled d - 2d scan), involvedthe comparison of the intensities of an X-ray powder pattern to those ofauthentic powder sample. The second method, a somewhat simplerprocedure, involved the determination of thc ( I I I ) rocking curve for eachsubstrate. Both copper and silver were found to be heavily ( I I I ) textured(although less so than gold). Copper depositions conducted under poorvacuum conditions yielded random polycystalline surfaces. Character-ization of the gold substrates has been reported previously,36

X-ray Photoelectron Spectroscopy (XPS). Spcctra werc obtainedusing a Surface Science SSX-100 X-ray photoclcctron spectrometer that

Laibinis et al.

has previously been described.33 High-resolution spectra were obtained

at a pass en.tgy of 50 eV using a 600-pm spot size: survey spectra (2

tcant; were obtained at a pass energy of 100 eV and a 1000-pm spot size.

Use of peaks associated with the underlying substrate as refe-rencest

lAut4f , i , ) = 84.00 eV, Ag(3d5rz) = 368.3 .vt9u!?.plr) .= 932'7 eYlieads to'the unreasonable differdnces (range = 0.6 eV) in the position of

C( ls) associated wi th the monolayer which we bel ieve were due to

problems of calibration of our detector; spectra were instead referenced

io C{ ls) = 284 80 eV. At tenuat ion data were col lected at a pressure of-3 x 10-e Torr and a pass energy of 100 eV using a 1000-rrm spot size;

the slides were characterized in randonn order b-V obtaining a single scan

(*2 min) and i i t t ing the resul t ing spectra wi th an 807o Gaussianf 20Va

Lorentzian peak. Soectra of polycrystall ine docosanethiol required use

of a 2-eV flood gun t0 overcome charging; spectra were referenced to

C { l s ) = 2 8 4 . 8 0 e V .Contact Angles. Static advancing and receding contact angles were

measureil {jn \tallc drttps using a Ram€-Hart goniometer under ambient

condrtroq, . ( or i . : i l r0g i rquid.r uere appl ied (and rernoved) wi th a Matr ix

f cchnr. iosr* 'L

icrr r ' ; . rp ipet te operated at i ts iowest speed (- I p l is) '

Cont lct : rngi t ' \ r ' l i : r r re. is t t red on both s ides of the drop on at least 3

c l r , r r r r app i ; cC . r t d ; t f c r t ' n l pos i t i ons o f thc s l i de wh i ie the t i po f the p ipe t

ren ta ined a t tachedInfrared Spectroscopy. Thc -\.Decirometer. optics' and analytica! -PIg:

tocols used in this pup.riruu. been described in earlier publications.s'3?'5e

For the purposes of th is study, g iven the di f f icul ty of obtain ing,a c lean

referenci spectrum on either silver or copper, we have used two different

references:- ( I ) monolayers prepared from octadecanethiol-d37 on the

metal of interest as a reference mirror and (2) gold cleaned by oxidative

treatment in either peroxysulfuric acid or ozone under UV irradiation

(ABTEChI). (Caution: PeroxysulJuric acid reacts uiolently with or-

Eanic material.s and should be used with great care.ffi) This procedure

vietds a spccrrum for protonated samples in the 3000-cm-rregion whose

in tens , t i c . e r l r f l c re l i ab l v used ln the quan t i t a t i ve ana lys i s f ro rn wh ich

the ntr) reLui . l i , r i - r ,Jntat ions are 'deduccci . Al i Sp€ctra are reported as - !og

R R,. uhrrc Fl , ' lhc :cf lect iv j tv of lhc substrate wi th the monolayer and

R, . r : t he i c i l cc t t v t t i , r l t ne re f ' e rence . Spec t ra l s imu la t i ons u t i i i zed f i lm

ihrcknesses deterrnrned for the most part by s ingle wavelength el l ipso-

metr \ , . Several rndependent determinat ions were also made using spec-

troscopic cll ipsometry' over the wavelength 340-850 nm. The values used

fo r ca l cu la t i ons a re as fo l l ows : t l ) Au : C ,6 (19 .0 A ) , C , , (20 .3 A ) , C r t

(21 .6 A ) , C ,q (22 .9 A ) , C r (24 .0 A ) , and c r , (25 .5 A ) : (2 ) 49 : C16 (21 .5

A) , C , ' t (22 .8 A ) . C rE (24 .1 A ) , C re (25 .3 A ) , and c2e (26 .6 A ) ' We have

noted-earlier that ell ipsometric studies of silver samples are somewhat

problematic given the difficulty of obtaining reliable substrate constants.

The values reported above were obtained from replicate measurements

on sub-ctrates shrelderi from the iaboratory ambient. The values selected

we;"e. ; i i r reni i r ined in t l ie Resui ts. f ' rom ihe lower range of exper imental

veitr,:s. lnr,\ -(...-:esixrndrng rrcnsibty tt,the sizeS of the adsOrbate mOleCUleS

u3r rJ \ , , , , c , r r r : i r : ' ge r than rhe a l i - t rans ex tens ion o f the cha in . We

be i i cye the \e ya lues i r - - , bc accu ra tc to w i th in - * l A . T 'he wors t p ro -

lec t i on o l ' . . h rs g1 ' ; r - r1 r1 -1 lg ca lcu la t l ons o i t he cha in Or ien ta t iOn i s sma l l

rperhaps iess ihan l ' in r i on s i lver and gold) . The most important point

to note ls that the numbers used are but one parameter used to s imulate

the optical anisotroptes evidenced in the experimental data. Selection of

thickness values corresponding to an all-trans extension of the chains

would yield very similar quantitative fits to that shown above.

Note Added in Proof. Recent diffraction studies (X-ray andhelium) have shown that the lattice constant of n-alkanethiolatemonolayers supported on silver is shorter than that on gold. Theunit ceii of the former is -107o less (nearest neighbor spacingof 4.77 A). a result in very good agreement with that inferred frominfrared spectroscopy. The overlayer formed is an incommensurate

hexagonal phase. one reminiscent of an expanded 1{1 x

t1)ntO.9o structure. See: Fenter, P.; Eisenberger, P'; Li, J.;Camillone. S., I l l : Bernasek, S.; Scoles, G.; Ramanarayanan' T.A.: Liang, K. S. Langmuir, in Press.

The measured chain ti l ts on Au( I I I ) are also found to be ingood agreement with the values inferred from vibrational spec-troscopy: G. Scoles (personal communication).

Registry No. HD, 544-76-3; Cu, 7440-50-8; Ag, 7440-22'4; An,

(5E) Briggs. D.; Seah, M' P. Practical Surface Analysis; John Wiley:

Chichester,-i983; nppendix 4 and references cited therein.(59) Dubois, L. Fi.l Zegarski, B. R'; Nuzzo, R. G. Proc. Natl. Acad' Sci.

u.s.A, 1981, 84, 47 39-47 42.(60) Dobbs, D. A. l Bergman, R. G.;Theopold, K H--Qhey. .F!g 'Nrnt

1990. 6S ( I 7), 2. Erickson, C. Y . Chem' Eng. News t990' 68 (32), 2'

7440-57-5; CH3(CH), .SH (n = 3) ,513-53- l ; CH3(CH2)"SH (n = 4) ,l l0 -66-7; CH3(CH2)SH (n = 5) , l l l -31-9; CH3(CH2) 'SH (n = 6) ,1639-09-4: CH3(CH2),rSH (n = 7), I I l -88-6; CH3(CH2),,SH (n = 8),1455-21-6;CH3(CH2),.SH (n = 9), 143-10-2; CH3(CHz),SH (n = l0),5332-52-5; CH3(CH2),.SH (n = I l ) , I l2-55-0; CH3(CHJFH (n = l2\,19484-26-5; CH3(CH2),SH (n = l3),2079-95-0; CHr(CH2)'SH (n =l4), 25276-7G4; CHr(CHz),SH (n = I 5), 291 7-26-2:CH3(CH2)'SH (n= l6), 53193-22-9: CH3(CH2)'SH (n = l7), 2885-00-9; CH3(CH,),SH(n = l8), 53193-23-0; CH3(CH2),.SH (n = l9), 13373-97-2: CHr

7 t67

(CH2),SH (n = 20), 66326-t7-8; CHr(CHz)SH (n -- 2l ), 7773-83-3.

Supplementery Materiel Aveileble: Table of wetting propertiesof monolayers prepared from different solvents, IR spectra oflow-quality monolayers on copper, plot of intensity of CH2 modeswith chain length, and discussion of the methods used in simulatingIR spectra (l I pages). Ordering information is given on anycurrent masthead page.